fix: CVE-2026-42600 remove ReadMultiple storage-REST API

The internode storage-REST ReadMultiple endpoint (/rmpl) joined
attacker-controlled Bucket/Prefix/Files into a filesystem path with no
validation, letting a peer with internode credentials read files outside
the drive root (GHSA-xh8f-g2qw-gcm7).

ReadMultiple has had no production caller since upstream #20390 removed
the last one (listParts) in Sep 2024; multipart now uses ReadParts (/rps).
Following the upstream fix, remove the whole API instead of validating
paths: route constant and registration, server handler, REST client
wrapper, the StorageAPI/xlStorage/xlStorageDiskIDCheck methods, the
storageMetricReadMultiple metric, and the ReadMultipleReq/Resp datatypes.
Regenerated the msgp and stringer outputs; storageRESTVersion stays at v63.

Co-authored-by: Codex <codex@openai.com>
Co-authored-by: Claude Code <claude-code@anthropic.com>

cmd/storage-rest-common.go

@@ -41,7 +41,6 @@ const (
 	storageRESTMethodRenameFile     = "/rfile"
 	storageRESTMethodVerifyFile     = "/vfile"
 	storageRESTMethodStatInfoFile   = "/sfile"
-	storageRESTMethodReadMultiple   = "/rmpl"
 	storageRESTMethodCleanAbandoned = "/cln"
 	storageRESTMethodDeleteBulk     = "/dblk"
 	storageRESTMethodReadParts      = "/rps"