Fix missing lock around _controllers access. (#28384)

The rules for the locking are that we must lock when on the Matter queue, which
is the case in createControllerOnExistingFabric.

src/darwin/Framework/CHIP/MTRDeviceControllerFactory.mm

@@ -96,7 +96,7 @@ @interface MTRDeviceControllerFactory ()
 //
 // 1) The only mutating accesses to the controllers array happen when the
 // current queue is not the Matter queue. This is a good assumption, because
-// the implementation of the fucntions that mutate the array do sync dispatch
+// the implementation of the functions that mutate the array do sync dispatch
 // to the Matter queue, which would deadlock if they were called when that
 // queue was the current queue.
 // 2) It's our API consumer's responsibility to serialize access to us from
@@ -597,7 +597,11 @@ - (MTRDeviceController * _Nullable)createControllerOnExistingFabric:(MTRDeviceCo
  return;
  }
 
- for (MTRDeviceController * existing in _controllers) {
+ os_unfair_lock_lock(&_controllersLock);
+ NSArray<MTRDeviceController *> * controllersCopy = [_controllers copy];
+ os_unfair_lock_unlock(&_controllersLock);
+
+ for (MTRDeviceController * existing in controllersCopy) {
  BOOL isRunning = YES; // assume the worst
  if ([existing isRunningOnFabric:fabricTable fabricIndex:fabric->GetFabricIndex() isRunning:&isRunning]
  != CHIP_NO_ERROR) {