fix(mcp): add timeout to background thread join in stop() to prevent hangs

[giulio-leone]

Summary

When an Agent holding an MCPClient is created inside a function and goes out of scope, the Agent.__del__ finalizer calls MCPClient.stop() which invokes _background_thread.join(). If the background thread cannot exit promptly (e.g. transport subprocess teardown is slow), join() blocks indefinitely, causing the entire process to hang on exit.

Changes

• Add a timeout (equal to startup_timeout, default 30s) to the join() call in stop()
• If the thread does not exit within the timeout, log a warning and proceed with cleanup
• The thread is already a daemon thread (daemon=True), so it will be cleaned up by the interpreter on process exit

Testing

• Updated existing tests to verify join() is called with timeout
• Added new test test_stop_does_not_hang_when_join_times_out covering the timeout path

Fixes #1732

[Copilot]

Pull request overview

Prevents process hangs on interpreter shutdown by ensuring MCPClient.stop() doesn’t block indefinitely when joining the background thread (notably when called from Agent.__del__).

Changes:

• Add a timeout (based on startup_timeout) to _background_thread.join() in MCPClient.stop()
• Log a warning when the join times out and continue cleanup
• Update/add tests to assert join(timeout=...) is used and exercise the timeout path

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/strands/tools/mcp/mcp_client.py	Adds join(timeout=...) + warning when the background thread doesn’t exit promptly.
tests/strands/tools/mcp/test_mcp_client.py	Updates existing stop() tests for the new join timeout behavior and adds a new timeout-path test.

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[giulio-leone]

All CI checks pass. Ready for review.