* fix(kms): drop the KmsClient trait import removed by #5501
The local backup export tests (#5499) merged after #5501 folded the
KmsClient trait into KmsBackend, leaving a dead trait import that breaks
'cargo test -p rustfs-kms' compilation on main. create_key is an inherent
method on LocalKmsClient since #5501, so the import is unnecessary.
* fix(kms): CAS Transit metadata writes and bound the metadata cache
Transit KV metadata writes were whole-record overwrites with no
precondition, so two nodes mutating the same key could silently clobber
each other's lifecycle state, and the process-local metadata cache had
neither a TTL nor a capacity bound, so a key disabled or scheduled for
deletion on one node stayed usable on every other node until restart.
- Replace write_metadata_to_kv with a versioned read
(read_metadata_from_kv_versioned) plus a check-and-set write
(cas_write_metadata_to_kv); mutate_key_metadata re-reads the
authoritative record and re-runs the state gate on every attempt, and a
lost CAS race retries with a fresh snapshot (bounded budget) instead of
replaying the stale one.
- Migrate every read-modify-write caller: enable, disable, schedule and
cancel deletion, rotate version bump, the expired-key tombstone, and
both create paths (create-only CAS that read-confirms the winner on a
lost race).
- Bound the metadata cache with moka (300s TTL, 1024 entries) and drop a
key's entry when a transit data call reports it gone server-side.
- Fail closed when the synthesized-metadata fallback cannot be read or
persisted: the fabricated Enabled record is only served after a durable
create-only CAS write, closing the gate weakening documented as a KNOWN
RISK; the persistence fallback for pre-metadata keys is kept.
Refs rustfs/backlog#1581 (part of rustfs/backlog#1562)
