* s3: return the multipart object checksum in the CompleteMultipartUpload body
S3 carries the flexible-checksum members of CompleteMultipartUploadResult in
the XML body, not in response headers, so every SDK read back an empty
checksum from an upload that asked for one.
Claude-Session: https://claude.ai/code/session_01Huux1uh7JxAbf8yypMYrMk
* s3: echo the checksum algorithm and type from CreateMultipartUpload
The upload directory already records both, but the response dropped them, so a
client could not confirm which checksum its parts had to carry.
Claude-Session: https://claude.ai/code/session_01Huux1uh7JxAbf8yypMYrMk
* test: multipart upload reports the object checksum it was asked for
Covers every algorithm end to end: the create response echoes the algorithm and
type, the complete response carries the checksum, and it matches what a later
HEAD reports.
Claude-Session: https://claude.ai/code/session_01Huux1uh7JxAbf8yypMYrMk
* s3: preserve checksums for copied multipart parts
* s3: return checksums from multipart copy
* s3: pin the upload's checksum algorithm on copy-part re-stream
* s3: note why UploadPartCopy uses the re-stream slow path
* s3: explain the TLS proxy in the multipart copy checksum test
* s3: cover nil and unknown-algorithm edge cases in copy checksum tests
* s3: cover all checksum algorithms in the multipart copy test
* s3: run all checksum integration tests, not just presigned
Chris Lu