fix(bigtable): surface swallowed MutationsBatcher flush errors

[mutianf]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request deprecates the background timer-based flush in MutationsBatcher and updates the close() method to handle exceptions during the final synchronous flush, ensuring that executor shutdown and cleanup are not aborted. The reviewer pointed out that catching a generic Exception in close() could lead to nested MutationsBatchError exceptions, and suggested explicitly catching and unpacking MutationsBatchError to maintain a flat list of exceptions.