feat(models): add BlockChunk type to chat.{start,append,stop}Stream methods

[srtaalej]

Summary

This PR adds BlocksChunk model class to support the new blocks chunk type in the streaming API (chat.startStream, chat.appendStream, chat.stopStream). Follows the April 16, 2026 changelog which notes updates to the text-streaming API methods to support streaming blocks.

Testing

Sample app.py

from slack_sdk.models.blocks import CardBlock, CarouselBlock                                                                                                                                                                
from slack_sdk.models.messages.chunk import BlocksChunk                                                                                                                                                                     
                                                                                                                                                                                                                              
  streamer = client.chat_stream(                                                                                                                                                                                              
      channel="C0123456789",                                                                                                                                                                                                  
      thread_ts="1700000001.123456",                                                                                                                                                                                          
      recipient_team_id="T0123456789",                                                                                                                                                                                        
      recipient_user_id="U0123456789",                                                                                                                                                                                        
  )                                                                                                                                                                                                                           
                                                                                                                                                                                                                              
  streamer.append(markdown_text="Here's a **card** streamed via `BlocksChunk`:\n\n")                                                                                                                                          
                                                                                                                                                                                                                              
  streamer.append(                                                                                                                                                                                                            
      chunks=[    
          BlocksChunk(                                                                                                                                                                                                        
              blocks=[                                                                                                                                                                                                        
                  CardBlock(                                                                                                                                                                                                  
                      title="Streaming Card",                                                                                                                                                                                 
                      subtitle="Sent via BlocksChunk",                                                                                                                                                                        
                      body="This card was streamed inside a chunks array.",                                                                                                                                                   
                  ),                                                                                                                                                                                                          
              ]                                                                                                                                                                                                               
          ),                                                                                                                                                                                                                  
      ]                                                                                                                                                                                                                       
  )               
                                                                                                                                                                                                                              
  streamer.stop(                                                                                                                                                                                                              
      chunks=[
          BlocksChunk(                                                                                                                                                                                                        
              blocks=[                                                                                                                                                                                                        
                  CarouselBlock(                                                                                                                                                                                              
                      elements=[                                                                                                                                                                                              
                          CardBlock(title="Card 1", subtitle="First", body="First card in carousel."),                                                                                                                        
                          CardBlock(title="Card 2", subtitle="Second", body="Second card in carousel."),                                                                                                                      
                          CardBlock(title="Card 3", subtitle="Third", body="Third card in carousel."),                                                                                                                        
                      ]                                                                                                                                                                                                       
                  ),                                                                                                                                                                                                          
              ]                                                                                                                                                                                                               
          ),      
      ]                                                                                                                                                                                                                       
  )           

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs (Documents)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.15%. Comparing base (73d255a) to head (3c0d3ba).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1872      +/-   ##
==========================================
+ Coverage   84.09%   84.15%   +0.06%     
==========================================
  Files         117      117              
  Lines       13325    13337      +12     
==========================================
+ Hits        11205    11224      +19     
+ Misses       2120     2113       -7     

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[zimeg]

@srtaalej Nice! LGTM with a note on alphabetics I'd like us to keep pattern for since it makes later finding and additions less difficult I think!

[zimeg]

🧮 quibble: Might this be alright to move up for alphabetical ordering to match other entries?