feat(pubsub): support batch mode in WriteToPubSub transform

[liferoad]

Add support for batch mode execution in WriteToPubSub transform, which previously only worked in streaming mode. Update documentation and add tests to verify batch mode functionality with and without attributes.

Fixes #35990

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.80%. Comparing base (a4ad5cd) to head (3b7e445).
⚠️ Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
...ache_beam/runners/dataflow/ptransform_overrides.py	65.21%	8 Missing ⚠️
sdks/python/apache_beam/io/gcp/pubsub.py	92.10%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #36027      +/-   ##
============================================
+ Coverage     56.79%   56.80%   +0.01%     
  Complexity     3385     3385              
============================================
  Files          1220     1220              
  Lines        185081   185147      +66     
  Branches       3508     3508              
============================================
+ Hits         105110   105167      +57     
- Misses        76646    76655       +9     
  Partials       3325     3325              

Flag	Coverage Δ
python	81.00% <83.33%> (+<0.01%)	⬆️

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[scwhittle]

Thanks! A few comments but I think this approach leaves the code in a more readable state.

[scwhittle]

why is there a flush timeout? Completing processing without waiting for all of the messages to be consumed by pubsub could lead to data loss

[liferoad]

I added the timeout exception, which should trigger Dataflow to retry for batch jobs. The idea is to avoid any stuckness when publishing the messages.

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