copy: support metadata overrides on the large-file copy path

[goanpeca]

Summary

Ensure metadata replacement for copy also works when the action chooses the SDK's copyLargeFile path.

Why

#108 covers metadata replacement for server-side copy, but explicitly leaves large-file copy metadata handling out of scope. The SDK's copyLargeFile accepts destination contentType and fileInfo, so large objects should not lose feature parity with small objects just because they cross the multipart threshold.

SDK support (already available)

• Bucket.copyLargeFile({ contentType, fileInfo })
• Bucket.copyFile({ metadataDirective, contentType, fileInfo })
• CopyFileRequest.metadataDirective

Proposed surface

After #108 defines metadata-directive, content-type, and file-info, forward the corresponding destination metadata to copyLargeFile when the source size requires multipart copy.

Acceptance criteria

• Large-file copy can set destination contentType.
• Large-file copy can set destination fileInfo.
• Small-file metadata replacement remains covered by copy: support metadata-replace (content-type and file-info override on server-side copy) #108.
• Behavior is documented so users know metadata override applies regardless of object size.
• Tests cover both small-copy and large-copy metadata behavior.

Out of scope

• Server-side encryption on copy, tracked in copy: support server-side encryption (SSE-C source key and destination SSE) #96.
• Copy byte ranges, tracked separately.