CopyObjectResult omits checksum fields and minio-go drops them

[Vonng]

Problem

SILO persists CopyObject checksums and exposes them through HEAD/GET, but its successful CopyObject XML response contains only LastModified and ETag. It omits the checksum elements defined by the S3 CopyObjectResult contract.

An explicit CRC32 copy currently returns:

<CopyObjectResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <LastModified>...</LastModified>
  <ETag>...</ETag>
</CopyObjectResult>

even though the destination object has a stored CRC32 checksum.

The gap is inherited from archived upstream MinIO:

• cmd/api-response.go CopyObjectResponse has only ETag and LastModified.
• generateCopyObjectResponse receives no ObjectInfo/checksum data.

There is also a downstream parsing gap in active minio-go:

• copyObjectResult has only ETag and LastModified.
• CopyObject returns UploadInfo without populating its existing checksum fields.

Relationship to #63

#63 fixes which bytes are covered by the stored checksum and explicitly verifies HEAD/GET. This issue is a separate public response-contract change and should not be folded into #63.

Proposed direction

SILO server

1. Add the five checksum fields currently supported by SILO plus ChecksumType to CopyObjectResponse.
2. Populate them from the committed destination ObjectInfo after decrypting checksum metadata with the request headers.
3. Keep omitempty behavior for objects without checksum metadata.
4. Register the exported response fields in the compatibility baseline.

minio-go

1. Add checksum fields and ChecksumType to copyObjectResult.
2. Copy them into the checksum fields already present on UploadInfo.
3. Add XML parsing and CopyObject response tests.

MCLI and SILO Console currently ignore the CopyObject UploadInfo checksum fields, so they require no behavioral change; they can consume the corrected library result later without a wire change.

Acceptance criteria

• CopyObjectResult returns the correct CRC32, CRC32C, CRC64NVME, SHA1, or SHA256 value when present.
• ChecksumType is FULL_OBJECT for single-operation copies, including multipart-composite sources.
• Default CRC64NVME is returned for copied objects that had no source checksum.
• SSE-C and SSE-S3 responses return the logical checksum without exposing encrypted metadata.
• No-checksum legacy cases remain omitted through omitempty.
• minio-go CopyObject returns the same values in UploadInfo.

[Vonng]

The server-side defect was resolved by #70 in f2520f334. CopyObjectResult now returns the checksum fields committed for the destination object, with regression coverage for supported algorithms and the no-checksum case.

Client-side parsing has also been resolved upstream by minio-go PR #2295, merged as b8b8f258e; minio-go#2294 is closed. Both known source-level defects are therefore fixed.

Delivery remains a separate gate: no tagged minio-go release currently contains that merge, and SILO, SILO Console, and MCLI still pin v7.0.99. The latest SILO main CI is green, but neither the SILO server fix nor a consuming minio-go release has shipped in a new tagged SILO release.