fix(bulk_writer): propagate sync commit failures

[SiluPanda]

Summary

Fixes #2314.

This change makes synchronous bulk-writer commits propagate failures from the background flush thread instead of only logging them and returning normally.

The current implementation starts _flush() on a thread, joins it for the default synchronous commit(), and then resets the buffer state. If persist() or a remote upload callback fails, the exception stays inside the worker thread, so callers see a successful return and may continue with an empty batch_files result.

The patch records the first flush exception and re-raises it on the caller thread:

• after waiting for any previous flush to finish, before starting a new one
• after joining the synchronous flush thread

This keeps the existing asynchronous flush model, but makes the default synchronous LocalBulkWriter.commit() and RemoteBulkWriter.commit() fail loudly and become catchable by user try/except blocks.

This also covers the same root cause reported in #2628 for RemoteBulkWriter.commit().

Testing

• pytest -q tests/unit/test_local_bulk_writer.py tests/unit/test_remote_bulk_writer.py
• make lint
• manual reproduction with patched failures now raises on the caller thread for both local persist failures and remote upload failures

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[SiluPanda]

Closing this for now: this repository enforces DCO sign-off, and I cannot provide or maintain that legal attestation for this contribution. The patch remains available on the branch for maintainers or a human contributor to reuse if helpful.