fix/mcp_session_auto_close_when_Mcpworkbench_deleted

[SongChiYoung]

Why are these changes needed?

As-is: Deleting McpWorkbench does not close the McpSession.
To-be: Deleting McpWorkbench now properly closes the McpSession.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[jackgerrits]

Do you think this might fix #6467?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.75%. Comparing base (6427c07) to head (ec8358b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6497      +/-   ##
==========================================
- Coverage   78.75%   78.75%   -0.01%     
==========================================
  Files         225      225              
  Lines       16581    16589       +8     
==========================================
+ Hits        13058    13064       +6     
- Misses       3523     3525       +2     

Flag	Coverage Δ
unittests	78.75% <100.00%> (-0.01%)	⬇️

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[SongChiYoung]

While attempting to improve code coverage, I discovered that the previous fallback logic in del (used to stop the actor if McpWorkbench is GC’ed) did not behave correctly under certain loop conditions.

As a result, the shutdown logic has been rewritten.

This change simplifies the behavior, avoids misleading fallback attempts, and ensures the shutdown path is verifiable via testing (e.g., pytest.warns()).

[SongChiYoung]

@jackgerrits
Regarding #6467
This change might help in some scenarios, especially when McpWorkbench is deleted unintentionally (e.g., via GC or teardown).

That said, I’m not entirely confident about the lifecycle behavior on Windows — I’m not a specialist in how del interacts with the asyncio loop on that platform.

Ideally, McpWorkbench should be used via async with, in which case aexit guarantees proper shutdown of the MCP session.

The fact that it doesn’t close cleanly in #6467 is quite weird and deserves separate investigation.

[jackgerrits]

Sounds good, thanks!