Touch expires key in SortedSetRedisSessionExpirationStore

[lsh1215]

SortedSetRedisSessionExpirationStore#cleanupExpiredSessions touches the session key (<namespace>:sessions:<id>) instead of the expires shadow key (<namespace>:sessions:expires:<id>).

RedisIndexedSessionRepository#onMessage only handles keyspace events whose key starts with the sessions:expires: prefix. Because the store touches the session key, expiration does not produce an event that matches that prefix, so SessionDeletedEvent and SessionExpiredEvent are not published for sessions tracked by this store. The principal index cleanup and the removal from the expiration sorted set run from that same onMessage handler, so they do not happen either.

This affects RedisIndexedSessionRepository when it is configured with SortedSetRedisSessionExpirationStore. MinuteBasedRedisSessionExpirationStore and the reactive SortedSetReactiveRedisSessionExpirationStore both touch the expires key and are not affected.

Change

cleanupExpiredSessions now touches <namespace>:sessions:expires:<id>, which matches the prefix that onMessage listens on and is consistent with the other two stores. The existing private helper was renamed from getSessionKey to getExpiredKey to reflect the key it builds and to match the naming used in RedisIndexedSessionRepository and ReactiveRedisIndexedSessionRepository.

Testing

• Updated SortedSetRedisSessionExpirationStoreTests#cleanupExpiredSessionsThenTouchExpiredSessions to assert that the expires key is touched. It fails before the change and passes after.
• ./gradlew :spring-session-data-redis:test passes.
• ./gradlew :spring-session-data-redis:integrationTest --tests "*SortedSetRedisSessionExpirationStoreITests" passes.
• checkstyle and Spring Java Format checks pass for the changed files.

Closes gh-3468

cc @rwinch