[Python] Finish async bundles after processing outputs

[bvolpato]

On the thread-pool path, AsyncWrapper calls finish_bundle() before consuming the wrapped process() result. A buffered transform can flush an empty bundle while the runner sees the input as complete.

This change consumes process outputs before finalization and flattens iterable returns to match the DoFn contract. If iteration fails, finalization is skipped and the input remains available for retry. Strings and bytes keep their existing scalar behavior. Returned lists and tuples now emit their elements individually, so code relying on the previous extra nesting will see a different output shape.

Local regression validation passed. A DirectRunner pipeline with a SQLite sink loses its buffered write with the unfixed implementation and preserves it with this change; the asyncio path passes as a control.

Run the Python unit suite with:

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

---

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• Replace these instructions with a description of your change and why you have made it.
• Mention the appropriate issue in your description (for example: addresses #123 or fixes #456).
• Update CHANGES.md with noteworthy changes.

See CONTRIBUTING.md for contribution guidelines.

[bvolpato]

Run Python_ML PreCommit 3.12

[bvolpato]

Run Python_ML PreCommit 3.13

[bvolpato]

Run Python_ML PreCommit 3.12

[bvolpato]

Run Python_ML PreCommit 3.12

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.25%. Comparing base (76f6e6d) to head (388aacb).
⚠️ Report is 143 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/transforms/async_dofn.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40000      +/-   ##
============================================
+ Coverage     56.16%   56.25%   +0.08%     
  Complexity     2288     2288              
============================================
  Files          1124     1124              
  Lines        178336   178378      +42     
  Branches       1489     1489              
============================================
+ Hits         100170   100343     +173     
+ Misses        75645    75514     -131     
  Partials       2521     2521              

Flag	Coverage Δ
python	79.72% <88.88%> (+0.16%)	⬆️

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[github-actions]

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R: @tvalentyn for label python.

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