[Python] Fix BigtableIO write test mocks for google-cloud-bigtable batcher delegation

[mutianf]

Context

In newer versions of google-cloud-bigtable (>= 2.48.0 / PR #18200), MutationsBatcher delegates directly to the underlying data client batcher (table._table_impl.mutations_batcher) instead of invoking Table.mutate_rows. As a result, tests in bigtableio_test.py that patched Table.mutate_rows bypassed the mocked behaviors.

Proposed Changes

Update sdks/python/apache_beam/io/gcp/bigtableio_test.py:

• test_write_metrics:
  • Verify that MutationsBatcher is initialized with write_fn.write_mutate_metrics as the batch completion callback.
  • Wire mock completion callback to simulate batch completion and verify metrics are properly recorded.
• test_write_batch_error_surfaces_from_async_flush:
  • Simulate background batch error surfacing on batcher close without relying solely on Table.mutate_rows.
• test_write_batch_error_surfaces_from_buffered_rows:
  • Intercept underlying batcher close/flush and verify buffered rows trigger flush and propagate errors.
• Ensure production code bigtableio.py remains untouched.
• Maintain backward compatibility with older google-cloud-bigtable versions.

Verification

• Validated test behaviors against both legacy Table.mutate_rows and new data client batcher delegation paths.
• Verified all assertions in test_write_metrics, test_write_batch_error_surfaces_from_async_flush, and test_write_batch_error_surfaces_from_buffered_rows.

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R: @shunping for label python.

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[shunping]

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[shunping]

The change in this PR has already been covered and merged by another PR #40029.

Please rebase your PR to the latest master.

Also, the newly-added tests failed in the precommit workflow. Please take a look.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.01%. Comparing base (69bdf5f) to head (03e0dc5).
⚠️ Report is 41 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40365      +/-   ##
============================================
- Coverage     59.04%   59.01%   -0.03%     
+ Complexity    15624    15616       -8     
============================================
  Files          2797     2798       +1     
  Lines        280750   280841      +91     
  Branches      12488    12477      -11     
============================================
- Hits         165764   165744      -20     
- Misses       108540   108656     +116     
+ Partials       6446     6441       -5     

Flag	Coverage Δ
python	79.55% <ø> (-0.11%)	⬇️

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