Fix in-process Execution API loop stopped while its transport is in use

[potiuk]

The in-process Execution API owns a background asyncio event loop + daemon thread. Their cleanup was registered with weakref.finalize keyed on the InProcessExecutionAPI instance. But callers build a sync Client from InProcessExecutionAPI().transport and discard the factory object, so the instance was garbage-collected while the transport was still in use. The finalizer then stopped the loop, and every subsequent request hung on the dead loop -- surfacing as Timeout failures across the Dag-processor and triggerer in-process API tests on main.

Tie the loop + thread + lifespan cleanup to the lifetime of the WSGI transport (what callers actually retain) instead of the factory instance. This keeps the original thread-leak fix intact -- the loop is still stopped and the thread joined once nothing can use the transport -- while no longer tearing it down prematurely.

Follow up after #68840

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[seanghaeli]

Hi @potiuk, thanks for the fix, sorry for jumping in, I assume you're asleep so I made another PR for the full fix so it can be merged now, needed it urgently for downstream PRs.

[potiuk]

Fixed by #68865 copied from this one