Bug Fixes

• Fixed cache backfill: In-memory cache is now repopulated when data is retrieved from file cache after expiration
• Fixed thread safety in InMemoryCachingService: Now uses ConcurrentDictionary and proper locking for multi-threaded environments
• Fixed missing locking in FileCachingService.StoreLanguagesAsync: Concurrent writes no longer corrupt the languages cache file
• Fixed file sharing: ReadAllTextAsync now allows concurrent reads (FileShare.Read)
• Fixed ClearCache: Now properly clears both translations and languages from in-memory cache

Improvements

• Resolved all nullable reference type warnings (CS8618)
• Added comprehensive thread-safety tests for caching services