Fix sync multipart upload error handling

[mstolarzblaxelai]

• Propagates sync multipart part-upload failures from worker threads.
• Aborts failed multipart uploads instead of completing with missing parts.
• Adds local regression coverage.

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Note

Adds exception propagation from worker threads in the sync multipart upload path. Previously, part failures were silently swallowed; now exceptions are collected under a lock and re-raised after all threads join, which triggers the existing _abort_multipart_upload cleanup in the outer except block.

^{Written by Mendral for commit edf972e.}

[mendral-app]

LGTM

The fix is correct. Thread safety is handled properly via results_lock for both results and exceptions writes. The raise exceptions[0] correctly propagates through the outer except block, ensuring _abort_multipart_upload is always called on failure. No issues found.

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