KAFKA-19971: Fix IllegalStateException from using entries after cache eviction

[nandini12396]

When the index cache is full and evicts entries, there's a race:

1. Entry marked for cleanup (markedForCleanup=true)
2. Index files renamed to .deleted
3. Entry may still be in Caffeine cache (async eviction)
4. cache.get() returns this marked entry
5. Cleanup runs after delay → cleanStarted=true
6. Subsequent lookups throw IllegalStateException

This fix detects entries with markedForCleanup=true in getIndexEntry(), invalidates them, and refetches from remote storage as a cache miss.