feat(mcp): surface MCP tool stream events in Agent.stream_async

[Ratish1]

Description

This PR enables streaming output from MCP tools to be surfaced through the Agent.stream_async() event stream.

This PR has the following changes:

• A new MCPClient.call_tool_stream method was introduced. It uses the progress_callback from the underlying mcp library to receive streaming chunks and yields them via an async generator.
• MCPAgentTool.stream was updated to call this new method, wrapping intermediate chunks in ToolStreamEvent and the final result in a ToolResultEvent.
• Unit tests were added for both MCPClient and MCPAgentTool to validate the new streaming functionality.

Related Issues

Closes #1273

Documentation PR

N/A

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare
• I ran unit tests

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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