[FIX] DockerCommandLineCodeExecutor multi event loop aware

[SongChiYoung]

Why are these changes needed?

Problem

Previously, in DockerCommandLineCodeExecutor, cancellation tasks were added directly to self._cancellation_tasks using asyncio.create_task():

self._cancellation_tasks.append(asyncio.create_task(self._kill_running_command(command)))

This caused issues when cancellation tasks were created from multiple event loops, leading to loop mismatch errors during executor shutdown.

Solution
This PR fixes the issue by introducing a dedicated internal event loop for managing cancellation tasks.
Cancellation tasks are now scheduled in a fixed event loop using asyncio.run_coroutine_threadsafe():

                    future: ConcurrentFuture[None] = asyncio.run_coroutine_threadsafe(
                        self._kill_running_command(command), self._loop
                    )
                    self._cancellation_futures.append(future)

Additional Changes

• Added detailed logging for easier debugging.
• Ensured clean shutdown of the internal event loop and associated thread.

Note
This change ensures that all cancellation tasks are handled consistently in a single loop, preventing cross-loop conflicts and improving executor stability in multi-threaded environments.

Related issue number

Closes #6395

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.

[codecov]

Codecov Report

Attention: Patch coverage is 71.73913% with 13 lines in your changes missing coverage. Please review.

Project coverage is 78.39%. Comparing base (63c791d) to head (daf1ff0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ext/code_executors/docker/_docker_code_executor.py	71.73%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6402      +/-   ##
==========================================
- Coverage   78.42%   78.39%   -0.03%     
==========================================
  Files         221      221              
  Lines       16062    16102      +40     
==========================================
+ Hits        12596    12623      +27     
- Misses       3466     3479      +13     

Flag	Coverage Δ
unittests	78.39% <71.73%> (-0.03%)	⬇️

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