Add startViewTransitionReadyFinished callback (#56461)

Summary:
Pull Request resolved: #56461

## Changelog:

[Internal] [Added] - Add startViewTransitionReadyFinished callback

Add `startViewTransitionReadyFinished()` to the view transition delegate so the reconciler can notify the native side when the ready callback's async work (e.g. spawned work from the `ready` promise `.then` - which includes invoking view transition event callbacks, starting animations from these callbacks) has completed. This was previously not capturable from the C++ side since `onReadyCallback` resolves the JS promise synchronously but the reconciler's follow-up work runs as a microtask.

- Add `startViewTransitionReadyFinished` to `UIManagerViewTransitionDelegate` and `ViewTransitionModule`
- Expose as a method on `nativeFabricUIManager` via `UIManagerBinding`
- Track `transitionReadyFinished_` state, reset to `false` before `onReadyCallback` and set to `true` when the reconciler calls back

Reviewed By: sammy-SC

Differential Revision: D99443648

fbshipit-source-id: 042f288c90d7548789185f7f0e46be73a12dcab4

Author: zeyap

packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

@@ -1092,6 +1092,24 @@ jsi::Value UIManagerBinding::get(
         });
   }
 
+  if (methodName == "startViewTransitionReadyFinished") {
+    return jsi::Function::createFromHostFunction(
+        runtime,
+        name,
+        0,
+        [uiManager](
+            jsi::Runtime& runtime,
+            const jsi::Value& /*thisValue*/,
+            const jsi::Value* /*arguments*/,
+            size_t /*count*/) -> jsi::Value {
+          auto* viewTransitionDelegate = uiManager->getViewTransitionDelegate();
+          if (viewTransitionDelegate != nullptr) {
+            viewTransitionDelegate->startViewTransitionReadyFinished();
+          }
+          return jsi::Value::undefined();
+        });
+  }
+
   if (methodName == "startViewTransition") {
     auto paramCount = 1;
     return jsi::Function::createFromHostFunction(

packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerViewTransitionDelegate.h

@@ -41,6 +41,8 @@ class UIManagerViewTransitionDelegate {
   {
   }
 
+  virtual void startViewTransitionReadyFinished() {}
+
   virtual void startViewTransitionEnd() {}
 
   struct ViewTransitionInstance {

packages/react-native/ReactCommon/react/renderer/viewtransition/ViewTransitionModule.cpp

@@ -353,7 +353,7 @@ void ViewTransitionModule::startViewTransition(
   }
 
   applySnapshotsOnPseudoElementShadowNodes();
-
+  transitionReadyFinished_ = false;
   if (onReadyCallback) {
     onReadyCallback();
   }
@@ -366,6 +366,10 @@ void ViewTransitionModule::startViewTransition(
   }
 }
 
+void ViewTransitionModule::startViewTransitionReadyFinished() {
+  transitionReadyFinished_ = true;
+}
+
 void ViewTransitionModule::suspendOnActiveViewTransition() {
   // Signal that the next transition should be suspended until the current
   // one finishes. The actual queueing happens in startViewTransition.

packages/react-native/ReactCommon/react/renderer/viewtransition/ViewTransitionModule.h

@@ -53,6 +53,8 @@ class ViewTransitionModule : public UIManagerViewTransitionDelegate,
       std::function<void()> onReadyCallback,
       std::function<void()> onCompleteCallback) override;
 
+  void startViewTransitionReadyFinished() override;
+
   void startViewTransitionEnd() override;
 
   std::optional<ViewTransitionInstance> getViewTransitionInstance(const std::string &name, const std::string &pseudo)
@@ -128,6 +130,8 @@ class ViewTransitionModule : public UIManagerViewTransitionDelegate,
 
   bool transitionStarted_{false};
 
+  bool transitionReadyFinished_{false};
+
   // When suspendNextTransition_ is true and a transition is active, the next
   // startViewTransition calls are queued instead of running immediately.
   bool suspendNextTransition_{false};