Add async disk cache existence check to avoid main thread blocking

[HarrisHan]

Summary

This PR addresses the main thread blocking issue (#2323) by adding asynchronous disk cache existence checking. The current implementation blocks the main thread when checking if an image exists in disk cache, causing frame drops in scrolling lists.

Problem

As reported in #2323:

• KingfisherManager.retrieveImage() performs disk cache checks on the calling thread
• When called from the main thread (common in UI code), this causes synchronous disk I/O
• Results in frame drops and scrolling performance issues, especially in SwiftUI

Solution

New APIs

1. DiskStorage.itemExists() - Lightweight existence check without loading data

  
 func itemExists(forKey key: String, referenceDate: Date = Date(), forcedExtension: String? = nil) -> Bool


 2. ImageCache.cachedType() - Async cache type checking
 public func cachedType(
     forKey key: String,
     processorIdentifier: String = DefaultImageProcessor.default.identifier,
     forcedExtension: String? = nil,
     callbackQueue: CallbackQueue = .mainCurrentOrAsync,
     completionHandler: @escaping @Sendable (CacheType) -> Void)

 Implementation Details

 - Memory cache check remains synchronous (fast, no I/O)
 - Disk cache check is dispatched to background queue
 - Results are delivered on specified callback queue
 - Maintains full backward compatibility

 Usage Example

 // Before (blocks main thread)
 let image = cache.retrieveImageInDiskCache(forKey: key)

 // After (non-blocking)
 cache.cachedType(forKey: key) { cacheType in
     switch cacheType {
     case .memory:
         // Image in memory
     case .disk:
         // Image on disk, load if needed
     case .none:
         // Not cached, download required
     }
 }

Testing

Added comprehensive tests in ImageCacheAsyncTests.swift:

• ✅ Correct cache type detection (memory/disk/none)
• ✅ Main thread non-blocking verification
• ✅ Performance testing with 1000+ items
• ✅ Thread safety validation

Performance Impact

• No impact on existing synchronous APIs
• Eliminates main thread blocking for existence checks
• Optimized disk check without loading full image data
• Maintains existing cache performance characteristics

Migration Guide

This is a non-breaking change. Existing code continues to work. For better performance:

  // Instead of checking synchronously
  if let _ = imageCache.retrieveImageInDiskCache(forKey: key) {
      // Image exists
  }

  // Use async check
  imageCache.cachedType(forKey: key) { type in
      if type != .none {
          // Image exists
      }
  }

Checklist

• Code compiles without warnings
• All existing tests pass
• Added new test coverage
• API follows Kingfisher conventions
• Thread-safe implementation
• Backward compatible