fix: return the remote part checksum to federated UploadPartCopy (#72)

The legacy etcd federation branch of CopyObjectPartHandler forwards copied
bytes with minio-go Core.PutObjectPart, which can only recover a checksum
from response headers. After the server-side part checksum work, the remote
computes and persists the checksum, but an AWS-compatible UploadPart response
correctly omits a checksum the request did not supply, so the proxy had
nothing to put in CopyPartResult.

The destination now returns the non-empty checksum fields of the PartInfo
produced by that exact write, but only when the request carries the
minio-federated application token that getRemoteInstanceClient already
attaches. Ordinary UploadPart responses are unchanged, and the checksum type
is deliberately not returned because UploadPart does not carry it. The
User-Agent is a response-shape hint only: it never gates authorization,
visibility or validation, and it can expose nothing beyond the checksum of
the body the caller just uploaded.

Reading the checksum from the same PartInfo that produced the response ETag
also keeps the pair bound to one write, so a concurrent overwrite of the same
part number cannot publish another writer's checksum.

Tests cover the application token gating matrix including lookalike tokens,
the real minio-go response parser, concurrent overwrites of one part number,
and an in-process two-deployment probe that drives the federation branch
through the real getRemoteInstanceClient into a real PutObjectPartHandler for
both FULL_OBJECT and COMPOSITE uploads.

Fixes #64

Signed-off-by: Feng Ruohang <rh@vonng.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

cmd/object-handlers.go

@@ -1122,6 +1122,12 @@ func getRemoteInstanceTransport() http.RoundTripper {
 	return nil
 }
 
+// federatedInternalAppName is the minio-go application token that
+// getRemoteInstanceClient attaches to every legacy federation proxy request. It
+// is declared next to its only producer so that the literal keeps its historical
+// file attribution in the rebrand compatibility baseline.
+const federatedInternalAppName = "minio-federated"
+
 // Returns a minio-go Client configured to access remote host described by destDNSRecord
 // Applicable only in a federated deployment
 var getRemoteInstanceClient = func(r *http.Request, host string) (*miniogo.Core, error) {
@@ -1136,7 +1142,7 @@ var getRemoteInstanceClient = func(r *http.Request, host string) (*miniogo.Core,
 	if err != nil {
 		return nil, err
 	}
-	core.SetAppInfo("minio-federated", ReleaseTag)
+	core.SetAppInfo(federatedInternalAppName, ReleaseTag)
 	return core, nil
 }