[Python] Wait for AsyncWrapper work before teardown

[bvolpato]

AsyncWrapper.teardown() can close the wrapped DoFn's resources while background work still uses them. A bounded DirectRunner pipeline reports DONE, then a delayed write fails with ValueError: I/O operation on closed file.

This change tracks work owned by each wrapper and waits for it before tearing down the wrapped DoFn. Asyncio tasks stay tracked through cancellation cleanup, and the shared executor and event loop remain available to other wrappers.

The AsyncWrapper tests and DirectRunner reproductions passed locally in both thread-pool and asyncio modes. Teardown has no timeout, so hung processing or cleanup can delay shutdown. Pending-output delivery and timer behavior are unchanged.

Run the Python unit suite with:

./gradlew :sdks:python:test-suites:tox:py312:testPython312

---

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• Update CHANGES.md with noteworthy changes.

See CONTRIBUTING.md for contribution guidelines.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.26%. Comparing base (76f6e6d) to head (93fa2ec).
⚠️ Report is 141 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40507      +/-   ##
============================================
+ Coverage     56.16%   56.26%   +0.09%     
  Complexity     2288     2288              
============================================
  Files          1124     1124              
  Lines        178336   178365      +29     
  Branches       1489     1487       -2     
============================================
+ Hits         100170   100354     +184     
+ Misses        75645    75490     -155     
  Partials       2521     2521              

Flag	Coverage Δ
python	79.74% <100.00%> (+0.18%)	⬆️

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