chore(bigtable): Added an internal batch completed callback to the data client mutations batcher

[daniel-sanche]

Migrating over @gkevinzheng PR from bigtable monorepo googleapis/python-bigtable#1308

Original description:

Changes made:

• Refactored logic from Table.mutate_rows from producing a list of Status protos from a MutationsExceptionGroup
• Added private keyword argument for a batch completion callback in the MutationsBatcher.
• Added unit tests/system tests.

Additional Changes:

• 504536b: incorporated gemini feedbck (guarded callback in try/catch, created distinct status objects)

Note to reviewers: This PR has already been reviewed and merged to a staging branch, with the intention of doing a single merge to main. We are now planning to slowly rollout these changes back to the main branch. Minimal re-review should be necessary

[gemini-code-assist]

Code Review

This pull request introduces a user batch completed callback to both the async and sync auto-generated MutationsBatcher classes, and refactors status-extraction logic into a shared helper function _get_statuses_from_mutations_exception_group in _helpers.py. The review feedback highlights critical issues with using list multiplication (e.g., [status_pb2.Status(...)] * size) which creates multiple references to the same mutable Status object, recommending list comprehensions instead. Additionally, the reviewer advises wrapping the user callback execution in a try-except block within the finally clause to prevent callback exceptions from masking other active exceptions, and suggests using module-level protobuf constants for consistency.