* Add md5 checksum validation support on PutObject and UploadPart
Per the S3 specification, when a client sends a Content-MD5 header, the server must compare it against the MD5 of the received body and return BadDigest (HTTP 400) if they don't match.
SeaweedFS was silently accepting objects with incorrect Content-MD5 headers, which breaks data integrity verification for clients that rely on this feature (e.g. boto3). The error infrastructure (ErrBadDigest, ErrMsgBadDigest) already existed from PR #7306 but was never wired to an actual check.
This commit adds MD5 verification in putToFiler after the body is streamed and the MD5 is computed, and adds Content-MD5 header validation to PutObjectPartHandler (matching PutObjectHandler). Orphaned chunks are cleaned up on mismatch.
Refs: https://github.com/seaweedfs/seaweedfs/discussions/3908
* handle SSE, add uploadpart test
* s3 integration test: fix typo and add multipart upload checksum test
* s3api: move validateContentMd5 after GetBucketAndObject in PutObjectPartHandler
* s3api: move validateContentMd5 after GetBucketAndObject in PutObjectHandler
* s3api: fix MD5 validation for SSE uploads and logging in putToFiler
* add SSE test with checksum validation - mostly ai-generated
* Update s3_integration_test.go
* Address S3 integration test feedback: fix typos, rename variables, add verification steps, and clean up comments.
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Co-authored-by: Chris Lu <chris.lu@gmail.com>
Michał Szynkiewicz
53048ffffb