fix(mcp): cancel workbench tool calls

[Luck (67-luck)]

Summary

• Fix McpWorkbench.call_tool passing a raw MCP coroutine to CancellationToken.link_future.
• Wrap the coroutine in an asyncio.Task before linking cancellation.
• Add an offline regression test covering cancellation of an in-flight tool call.

Fixes #8265.

Validation

• git diff --check
• python -m compileall for the changed source and test files
• The full pytest test cannot run in this environment because pytest and mcp are not installed.

[Luck (67-luck)]

@microsoft-github-policy-service agree