perf(s3vectors): parallelize put_vectors across batches using ThreadP…

[Uzmaa7]

Description

Parallelized batch execution in S3VectorsStore.put_vectors using ThreadPoolExecutor to speed up large vector ingestions while preserving error propagation.

Related issue

Fixes #13

Changes

• Wrapped 500-vector batch uploads in ThreadPoolExecutor with worker count bounded by batch count (min(max_workers, len(chunks))).
• Utilized as_completed and future.result() to ensure exceptions in any worker thread are re-raised immediately.
• Added throughput test with mock latency and error propagation unit test in tests/test_s3vectors.py.

Testing

• Tests pass locally
• Ruff checks pass
• Documentation updated, if applicable

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

[Uzmaa7]

I've submitted a pull request for this issue: #259. All CI checks are passing

[Kaap10]

LGTM! Clean implementation of batch parallelization using ThreadPoolExecutor with proper exception propagation and throughput unit tests.