fix(bigtable): Log warning on background task failure instead of exception

[daniel-sanche]

Bigtable data clients use background threads/coroutines to keep grpc channels warmed, and refresh them before expiration. These background tasks can fail due to resource exhaustion, or other issues.

This PR will log a warning and continue if there is an error during task set up, instead of rasing an exception

Also improves exception capture, which would sometimes state misleading information about the cause of the error.

[gemini-code-assist]

Code Review

This pull request updates the Bigtable client and query iterator to gracefully handle task creation failures during background channel refresh and instance registration by catching exceptions, logging warnings, and proceeding with the tasks disabled. The review feedback highlights that the newly added fallback unit tests should be updated to explicitly assert the expected call sequence and arguments of the mocked create_task helper, ensuring compliance with the repository's style guide on mock hygiene.