CombinePerKey with gbek (Python)

[damccorm]

As written right now, enabling the GBEK pipeline option doesn't work with CombinePerKey on runners which replace CombinePerKey. This is because CombinePerKey contains a GroupByKey; when runners replace this transform, they will either (a) notice that the transform structure doesn't match what they expect and fail (this is what the Dataflow runner does) or (b) silently replace the new GroupByEncryptedKey with whatever their default implementation is.

In fact, a naive optimized version of GroupByEncryptedKey cannot work at all since encryption happens before handing control to the runner, meaning that combiner lifting can't happen by default. For now, we'll work around this by not doing any lifting of CombinePerKey and using the default implementation, though this could be optimized in the future.

To achieve this, this PR simply doesn't return a URN for CombineValues when GBEK is used. This will cause all runners to use the Beam default implementation.

Example failing test before my changes to CombinePerKey: https://github.com/apache/beam/actions/runs/18232400391/job/51918702360. After my changes, this is succeeding. I'll do the same thing for Java once #36346 is in

Part of #36214

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.21%. Comparing base (6043660) to head (521b6d1).
⚠️ Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/transforms/core.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #36382      +/-   ##
============================================
- Coverage     36.22%   36.21%   -0.01%     
+ Complexity     1667     1666       -1     
============================================
  Files          1058     1058              
  Lines        165088   165213     +125     
  Branches       1190     1190              
============================================
+ Hits          59796    59840      +44     
- Misses       103116   103196      +80     
- Partials       2176     2177       +1     

Flag	Coverage Δ
python	40.53% <0.00%> (-0.01%)	⬇️

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Assigning reviewers:

R: @shunping for label python.

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[shunping]

LGTM!