Fix iOS PHImageManager crash from QoS background queue access (#1324)

PHImageManager methods were being called from QoS background queues (`QOS_CLASS_USER_INTERACTIVE`, etc.), violating Apple's thread safety requirements and causing crashes with `objc_storeWeak` deadlocks.

**Changes**

Dispatch all PHImageManager/PHCachingImageManager method calls to main queue:
- Image requests: `requestImageForAsset`, `requestAVAssetForVideo`
- Cache operations: `startCachingImagesForAssets`, `stopCachingImagesForAllAssets`
- Cancellation: `cancelImageRequest`

**Example**

Before (crashes on background queue):
```objc
- (void)fetchThumb:(PHAsset *)asset {
    PHImageRequestID requestId = [self.cachingManager requestImageForAsset:asset ...];
}
```

After (safe main thread execution):
```objc
- (void)fetchThumb:(PHAsset *)asset {
    dispatch_async(dispatch_get_main_queue(), ^{
        PHImageRequestID requestId = [self.cachingManager requestImageForAsset:asset ...];
    });
}
```

**Performance Impact**

No UI blocking occurs because PHImageManager API calls are non-blocking - they return immediately after submitting the request. Only the lightweight API call submission is dispatched to the main queue; the actual image processing work happens asynchronously on PHImageManager's own background threads.

QoS prioritization remains intact—background work still executes on appropriate queues, only the PHImageManager API calls are marshaled to the main thread.

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: Alex Li <[email protected]>

Author: Copilot

CHANGELOG.md

@@ -30,6 +30,8 @@ To know more about breaking changes, see the [Migration Guide][].
 
 ### Fixes
 
+- Fix PHImageManager crash on iOS by ensuring all PHImageManager/PHCachingImageManager methods are called on the main thread.
+  This resolves race conditions and deadlocks when thumbnail operations are dispatched to QoS background queues.
 - Fix EXC_BAD_ACCESS crash caused by accessing deallocated memory in async blocks on iOS.
   - Fixed PHCachingImageManager methods: `fetchThumb`, `exportAssetToFile`, `fetchFullSizeImageFile`.
   - Fixed PHAssetResourceManager methods: `fetchVideoResourceToFile`, `fetchOriginImageFile`.