feat: add mcp_servers field to app manifest

[zimeg]

Changelog

Add mcp_servers manifest field to support Slack MCP Client integration, allowing apps to declare MCP server connections with configurable authentication.

Summary

This pull request adds the mcp_servers field to the app manifest struct, enabling apps to configure MCP (Model Context Protocol) server connections.

The new MCPServer struct supports:

• url (required) — HTTPS endpoint of the MCP server
• auth_provider_key (optional) — reference to an external_auth_providers entry for OAuth
• auth_type (optional) — authentication method (no_auth, slack_identity_auth, manual_auth, dynamic_client_registration)

Schema definition: slackapi/manifest-schema#71

Preview

N/A — no UI changes.

Testing

• Verify mcp_servers is properly serialized/deserialized in JSON manifests with various auth configurations
• Confirm existing manifest tests still pass: go test ./internal/shared/types/

Notes

• The external_auth_providers field already exists; mcp_servers references it via auth_provider_key
• The use_pkce and token_url_config fields added in the schema PR are already supported here since oauth2 provider options use *RawJSON (passthrough)
• semver:patch — new field is additive and behind a feature flag on the platform side

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.69%. Comparing base (711bfee) to head (fe3cc7a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
+ Coverage   71.66%   71.69%   +0.03%     
==========================================
  Files         226      226              
  Lines       19148    19148              
==========================================
+ Hits        13723    13729       +6     
+ Misses       4216     4214       -2     
+ Partials     1209     1205       -4     

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