Apply the same fix as the previous commit to
createMultipartSSEKMSDecryptedReaderDirect: per-chunk SSE-KMS metadata
is validated upfront, but volume-server GETs fire lazily through
lazyMultipartChunkReader. At most one chunk body is live at any time.
This is the same eager-open-all-chunks shape that produced #8908's
truncated GETs for SSE-S3; SSE-KMS multipart objects with many chunks
were exposed to the same idle-keepalive failure mode under concurrent
load.
The wire format on disk is unchanged (same per-chunk metadata, same
encrypted bytes, same object Extended attributes). Existing SSE-KMS
multipart objects read back identically -- only when the volume-server
GETs fire changes.