fix(s3): validate x-amz-content-sha256 against received body (bug-hunt 5.4)

[vieiralucas]

Summary

Completes bug-hunt finding 5.4 at the correct layer. A plain (non-chunked) SigV4 PutObject carries the payload's real SHA-256 in the x-amz-content-sha256 header. fakecloud spooled the body but never compared this header against the bytes received, so a corrupt/tampered upload whose header disagreed with its body stored silently. AWS rejects the divergence with XAmzContentSHA256Mismatch (400).

Why the S3 layer, not sigv4

The earlier attempt (#2288) put this in sigv4 verification and was reverted — the signed payload is not reliably re-derivable there (aws-chunked / streaming empties the buffered body), so it wrongly rejected legitimate signed requests with IncompleteSignature. The S3 write handler holds the fully-decoded object bytes, so the hash is authoritative and there is no ambiguity.

Changes

• core: SpooledBody gains sha256_hex, computed in the same single streaming pass as md5_hex (no extra IO). Over the decoded payload for aws-chunked bodies.
• s3 write path: when x-amz-content-sha256 is a 64-char hex digest (not UNSIGNED-PAYLOAD, not a STREAMING-… marker, not aws-chunked), compare to the spool's sha256; return XAmzContentSHA256Mismatch on divergence.
• Markers / streaming skip the check entirely, so correct clients (SDK, aws-cli, boto3 — which default to aws-chunked STREAMING) are unaffected.

Test plan

• core unit: spool_computes_sha256_over_plain_payload, spool_sha256_is_over_decoded_aws_chunked_payload.
• e2e: s3_put_object_rejects_content_sha256_mismatch — bogus hex -> 400 XAmzContentSHA256Mismatch, correct hex -> 200, UNSIGNED-PAYLOAD -> bypass.
• regression guard (per the sigv4-body-hash lesson): iam_enforcement (48) + iam_enforcement_abac (9) — the FAKECLOUD_VERIFY_SIGV4=true paths fix(core,aws,s3): auth enforcement gaps (bug-hunt 5.1/5.3/5.4) #2288's revert restored, including the 3 tests the sigv4-layer version broke — all green.
• s3 (80), s3_aws_chunked, s3_streaming_body, and s3 conformance 52/52 green.

No non-code surface change: behavior now matches AWS more faithfully; invisible to correct clients, no new API/flag/field, no SDK change.

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Summary by cubic

Validate x-amz-content-sha256 against the received bytes for non‑chunked S3 PutObject requests. Mismatches now return 400 XAmzContentSHA256Mismatch to match AWS.

• Bug Fixes

  • Added sha256_hex to SpooledBody, computed alongside MD5 over the decoded payload.
  • In the S3 write path, when the header is a 64‑char hex and the request is not aws‑chunked, compare it to the spool hash and return XAmzContentSHA256Mismatch (400) on mismatch. The check runs in the S3 layer, not SigV4.
  • Skip checks for UNSIGNED-PAYLOAD, STREAMING-…, and aws‑chunked bodies, so streaming SDKs remain unaffected.

• Dependencies

  • Added sha2 (workspace).

^{Written for commit 99fa86b. Summary will update on new commits.}

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