Fix "Event loop is closed" race in EventLoopThread.force_stop

[nmingam]

Independent of any feature work and pre-exists on dev; surfaced while stabilizing CI.

tests/test_thread.py::test_thread_already_stopped intermittently errors at teardown with
RuntimeError: Event loop is closed (observed on the 3.11/3.13 CI legs; passes on 3.12/3.14 and locally).

Root cause: the worker thread's _thread_main finally runs self.loop.close() and then self.loop = None
as two separate statements. force_stop's if self.loop is None guard can therefore pass while the loop is
already closed, and the following call_soon_threadsafe raises.

Fix: snapshot self.loop into a local, also guard on loop.is_closed(), and swallow the RuntimeError
if the worker closes the loop after the check. The is_closed() guard is exercised by the existing
test_thread_already_stopped; the race-only except is marked # pragma: no cover.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (cd3378f) to head (19e86f5).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #727   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          61       61           
  Lines        4181     4183    +2     
=======================================
+ Hits         4162     4164    +2     
  Misses         19       19           

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