fix: resolve cache session handler index inconsistencies

[CodeWithKyrian]

Problem

The CacheSessionHandler could get into an inconsistent state where sessions exist in the cache but are missing from the session index, or vice versa. This happens due to:

• Partial write failures between index and session data
• Race conditions during garbage collection
• Cache backend inconsistencies or network issues

Solution

Added self-healing logic to the read() method that:

• Auto-repairs missing index entries: When a session exists in cache but not in index, automatically add it to the index
• Cleans up stale index entries: When index references a non-existent session, remove it from the index
• Maintains existing behavior: TTL checks and expiration logic remain unchanged

Changes

• Enhanced CacheSessionHandler::read() with consistency checks and auto-repair
• No breaking changes to the public API
• Backward compatible with existing session data

Testing

This fix is defensive and maintains all existing behavior while preventing inconsistent states that could cause session lookup failures.