[4.0.2 backport] CBG-5027 Move rev cache unlocks into defer

[adamcfraser]

CBG-5027

[Copilot]

Pull request overview

This PR refactors lock management in the LRU revision cache by consistently using defer statements for unlocks. The changes improve code safety by ensuring locks are released even if early returns or panics occur, reducing the risk of deadlocks. This is a backport to the 4.0.2 branch.

Key changes:

• Converted manual Unlock() calls to defer pattern across multiple functions
• Refactored Upsert() by extracting cache operations into a new upsertDocToCache() helper function
• Removed early return statements that required explicit lock releases