[iOS] Fix concurrent modification crash (#4008)

## Description

In `reattachHandlersIfNeeded` method we are iterating over handlers
contained in registry. The following crash was reported:

```
NSDictionary was mutated while being enumerated
```

This PR adds `@synchronized` on `registry._handlers` so attempting to
modify this value should be safe.

> [!NOTE]
> For now we have no consistent reproduction. I'm also not sure where
this concurrent modification takes place. It seems to be indirect, as
previous keyframe in provided stack trace was `flushOperations`.


## Test plan

Tested on the example code from #3964

Author: m-bert

packages/react-native-gesture-handler/apple/RNGestureHandlerManager.mm

@@ -250,15 +250,19 @@ - (void)reattachHandlersIfNeeded
   // display:none and siblings change, the native UIView backing a component may be recycled
   // and replaced. maybeBindHandler is a no-op if the view is nil or unchanged. This is only
   // needed for handlers using the old api.
-  for (RNGestureHandler *handler in _registry.handlers.objectEnumerator) {
-    if (handler.viewTag == nil || [handler usesNativeOrVirtualDetector]) {
-      continue;
-    }
+  NSDictionary *handlers = _registry.handlers;
+
+  @synchronized(handlers) {
+    for (RNGestureHandler *handler in handlers.objectEnumerator) {
+      if (handler.viewTag == nil || [handler usesNativeOrVirtualDetector]) {
+        continue;
+      }
 
-    [self maybeBindHandler:handler.tag
-             toViewWithTag:handler.viewTag
-            withActionType:handler.actionType
-          withHostDetector:nil];
+      [self maybeBindHandler:handler.tag
+               toViewWithTag:handler.viewTag
+              withActionType:handler.actionType
+            withHostDetector:nil];
+    }
   }
 }
 

packages/react-native-gesture-handler/apple/RNGestureHandlerRegistry.m

@@ -29,20 +29,29 @@ - (instancetype)init
 
 - (RNGestureHandler *)handlerWithTag:(NSNumber *)handlerTag
 {
-  return _handlers[handlerTag];
+  @synchronized(_handlers) {
+    return _handlers[handlerTag];
+  }
 }
 
 - (void)registerGestureHandler:(RNGestureHandler *)gestureHandler
 {
-  _handlers[gestureHandler.tag] = gestureHandler;
+  @synchronized(_handlers) {
+    _handlers[gestureHandler.tag] = gestureHandler;
+  }
 }
 
 - (void)attachHandlerWithTag:(NSNumber *)handlerTag
                       toView:(RNGHUIView *)view
               withActionType:(RNGestureHandlerActionType)actionType
             withHostDetector:(nullable RNGHUIView *)hostDetector
 {
-  RNGestureHandler *handler = _handlers[handlerTag];
+  RNGestureHandler *handler;
+
+  @synchronized(_handlers) {
+    handler = _handlers[handlerTag];
+  }
+
   RCTAssert(handler != nil, @"Handler for tag %@ does not exists", handlerTag);
   [handler unbindFromView];
   handler.actionType = actionType;
@@ -55,27 +64,43 @@ - (void)attachHandlerWithTag:(NSNumber *)handlerTag
 
 - (void)detachHandlerWithTag:(NSNumber *)handlerTag fromHostDetector:(RNGHUIView *)hostDetectorView
 {
-  RNGestureHandler *handler = _handlers[handlerTag];
-  if (handler.hostDetectorView != hostDetectorView)
+  RNGestureHandler *handler;
+
+  @synchronized(_handlers) {
+    handler = _handlers[handlerTag];
+  }
+
+  if (handler.hostDetectorView != hostDetectorView) {
     return;
+  }
+
   [handler unbindFromView];
 }
 
 - (void)dropHandlerWithTag:(NSNumber *)handlerTag
 {
-  RNGestureHandler *handler = _handlers[handlerTag];
+  RNGestureHandler *handler;
+
+  @synchronized(_handlers) {
+    handler = _handlers[handlerTag];
+    [_handlers removeObjectForKey:handlerTag];
+  }
+
   [handler unbindFromView];
-  [_handlers removeObjectForKey:handlerTag];
 }
 
 - (void)dropAllHandlers
 {
-  for (NSNumber *tag in _handlers) {
-    RNGestureHandler *handler = _handlers[tag];
-    [handler unbindFromView];
+  NSArray<RNGestureHandler *> *handlers;
+
+  @synchronized(_handlers) {
+    handlers = [_handlers allValues];
+    [_handlers removeAllObjects];
   }
 
-  [_handlers removeAllObjects];
+  for (RNGestureHandler *handler in handlers) {
+    [handler unbindFromView];
+  }
 }
 
 @end