Add content-idempotent path upload coordinator

[Hossain007]

Summary

Add a small synchronous coordinator for path uploads that compares full-object SHA-256 content before submitting an existing classic or CRT managed transfer.

The coordinator:

• hashes the local file in bounded chunks;
• calls HeadObject with checksum mode enabled;
• returns False without starting a transfer when content matches;
• creates a missing key with IfNoneMatch='*';
• replaces changed content with IfMatch set to the observed ETag;
• supplies the computed ChecksumSHA256 to the transfer;
• rejects caller-provided checksum and conditional-write arguments;
• propagates conditional conflicts without hidden retries; and
• raises a dedicated exception when an existing object has no comparable full-object SHA-256.

This PR intentionally supports paths only. It does not add a file-object API.

Stacked dependency

This draft is stacked on #371. The first commit is a rebase of that PR's conditional-write patch onto current develop; it is not a separate implementation of, or claim of authorship over, #371. After #371 merges, this branch should be rebased so the upstream diff contains only the coordinator and its tests.

The proposed Boto3 API is tracked in boto/boto3#4823.

Tests

• python -m pytest tests/unit tests/functional -q — 758 passed
• pre-commit run --all-files — passed
• Classic single-part and multipart requests assert the exact HEAD, PUT, and multipart completion parameters.
• CRT functional coverage verifies full-checksum conditional headers.
• Live CRT single-part and multipart integration tests are included but were not run locally because AWS credentials were unavailable.

The supported Python matrix and live AWS coverage remain CI/reviewer gates.

Development note

AI tools assisted with implementation and test iteration. Human review is pending while this PR remains a draft.