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.

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[scwhittle]

Could you rebase onto beam master? I would like to run some internal testing to verify Dataflow streaming update compatability but the import has conflicts due to the base.

Other than the minor comments things are looking good to me. However you might want to have a Python expert take a look to make sure things are done idiomatically since I'm not too familiar with Python.

[liferoad]

Could you rebase onto beam master? I would like to run some internal testing to verify Dataflow streaming update compatability but the import has conflicts due to the base.

Other than the minor comments things are looking good to me. However you might want to have a Python expert take a look to make sure things are done idiomatically since I'm not too familiar with Python.

Rebased.

[liferoad]

@damccorm for the Python review.

[damccorm]

Thanks!

[scwhittle]

Thanks! I tested manually that the update compatability for dataflow worked going from without -> with -> without this PR.