bigtable: Require google-cloud-bigtable>=2.42.0 and test Python BigtableIO write error surfacing

[mutianf]

• Bump the minimum google-cloud-bigtable to >=2.42.0,<3. 2.42.0 improves MutationsBatcher error handling so that errors raised during async flushes are surfaced instead of swallowed.
• Add _BigTableWriteFn write tests asserting that batch errors are surfaced (and thus fail the bundle, so the runner retries) from:
  • the async flush path,
  • the buffered-row flush at finish_bundle()/close(),
  • close() itself.
• Add a user agent in the python client

Previously a batch-level failure during an asynchronous flush could be dropped without failing the bundle. Requiring 2.42.0 and adding these regression tests guards the error-surfacing behavior end-to-end through Beam.

Fix #39755

[github-actions]

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R: @shunping for label python.

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