A 6 GiB object uploaded to the source as a 768-part multipart upload was
replicated to a generic S3 target with a single PutObject, and the target
rejected the body with EntityTooLarge. No CreateMultipartUpload was ever
issued, so the multipart replication transport never ran for the object
it exists for.
`replication_put_object_options` seeded the transport from
`object_info.is_multipart()` and then overwrote it with the second
return value of `decrypt_checksums`. Those two booleans do not mean the
same thing: the first is the object's storage shape, read from the ETag,
while the second reports whether the stored *checksum record* carries
per-part data. A full-object checksum -- what `aws s3 cp` writes by
default for a CRC algorithm -- is serialized with no MULTIPART flag even
on a multipart upload, so the record reports false and the object was
routed as a single PUT. `decrypt_checksums` documents this in
object_api/types.rs: callers that need routing must consult
`is_multipart()`. Replication did the opposite.
Route on the object's own shape, and let the checksum record only add
multipart-ness, never take it away. Objects already stored with such a
record are fixed too: the ETag was always right.
This also repairs the diagnosis of rustfs#6825, where the single-PUT
5 GiB guard fired against an object that was multipart all along and
told the operator to re-upload it as multipart.
Tests cover the three shapes the router has to separate: a multipart
object with a full-object checksum record (the regression, which fails
without this change), a multipart object with a composite record, and a
single-part object that must not be promoted onto multipart.
