[BugFix] Fix async core engine client finalizer

[njhill]

This fixes a bug where the asyncio event loop isn't properly obtained in the async engine client shutdown/finalizer logic.

(APIServer pid=769888)   File "/home/name/vllm_serve/.venv/lib/python3.12/site-packages/zmq/asyncio.py", line 116, in _default_loop
(APIServer pid=769888)     return asyncio.get_event_loop()
(APIServer pid=769888)            ^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=769888)   File "/usr/lib/python3.12/asyncio/events.py", line 702, in get_event_loop
(APIServer pid=769888)     raise RuntimeError('There is no current event loop in thread %r.'
(APIServer pid=769888) RuntimeError: There is no current event loop in thread 'MPClientEngineMonitor'

Fixes #24230.
Fixes #24305.

[gemini-code-assist]

Code Review

This pull request effectively resolves a critical bug in the asynchronous engine client's finalizer logic that caused a RuntimeError during shutdown. The fix correctly retrieves the asyncio event loop from the associated task, which is a much more robust approach than trying to get it from the current thread, especially within a finalizer that can be executed by the garbage collector. The additional safeguards, such as checking for a null loop and suppressing exceptions during task cancellation, further enhance the stability of the shutdown process. The changes are well-reasoned and correctly implemented.