Skip rewriting internal pcollection coders in a stage

[shunping]

Prior to the PR, unknown coders were potentially replaced (e.g., length-prefixed) for every transform within a processing stage. This approach, however, caused issues in Java pipelines (like #32931), where worker nodes required the original coder (e.g., SchemaCoder) to correctly relay data between internal transforms.

An observation driving this change is that only the input and output coders of a stage are critical for the runner, as actual serialization/deserialization occurs at these stage boundaries. Coders for internal transforms should be irrelevant to the runner's processing and will now be kept intact. This minimizes potential side effects from modifying the pipeline graph and ensures internal data flow remains correct.

#fixes #32931

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.59%. Comparing base (4ead940) to head (b38e32a).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #34655      +/-   ##
============================================
- Coverage     54.61%   54.59%   -0.03%     
  Complexity     1480     1480              
============================================
  Files          1008     1008              
  Lines        159758   159657     -101     
  Branches       1079     1079              
============================================
- Hits          87259    87162      -97     
+ Misses        70399    70397       -2     
+ Partials       2100     2098       -2     

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[lostluck]

Elegant! I thought for sure this would have required a fix Java side as well.

But i suppose if it doesn't happen in practice then what's to worry about.