Add inert Java plumbing for the Android pull model

Summary:
Java side of the pull model — dead code until the C++ wiring lands:
- `PullTransactionMountItem`: asks C++ (via `FabricUIManagerBinding.pullAndExecuteTransaction`) to pull the surface's pending transaction and apply it synchronously on the UI thread.
- `FabricUIManager.onTransactionAvailable` (JNI entry): enqueues a `PullTransactionMountItem`.
- `FabricUIManagerBinding.pullAndExecuteTransaction`: extern declaration.

One item is enqueued per notification, mirroring iOS; redundant items no-op in C++.

## Changelog:
[Android] [Added] - Add inert Java plumbing (`PullTransactionMountItem`, `FabricUIManager.onTransactionAvailable`) for the pull model

Differential Revision: D112309050

Author: Bartlomiej Bloniarz

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -27,6 +27,7 @@
 import androidx.annotation.AnyThread;
 import androidx.annotation.Nullable;
 import androidx.annotation.UiThread;
+import androidx.annotation.VisibleForTesting;
 import androidx.core.view.ViewCompat.FocusDirection;
 import com.facebook.common.logging.FLog;
 import com.facebook.infer.annotation.Assertions;
@@ -65,6 +66,7 @@
 import com.facebook.react.fabric.mounting.mountitems.MountItem;
 import com.facebook.react.fabric.mounting.mountitems.MountItemFactory;
 import com.facebook.react.fabric.mounting.mountitems.PrefetchResourcesMountItem;
+import com.facebook.react.fabric.mounting.mountitems.PullTransactionMountItem;
 import com.facebook.react.fabric.mounting.mountitems.SynchronousMountItem;
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags;
 import com.facebook.react.internal.featureflags.ReactNativeNewArchitectureFeatureFlags;
@@ -1009,6 +1011,26 @@ public void runGuarded() {
     }
   }
 
+  /**
+   * Pull model: called from C++ via JNI (usually on the commit thread) to signal that a transaction
+   * is available for {@code surfaceId}. Enqueues a PullTransactionMountItem so the UI thread pulls
+   * and applies the transaction itself, preserving mount-item ordering.
+   */
+  @SuppressWarnings("unused")
+  @AnyThread
+  @ThreadConfined(ANY)
+  @VisibleForTesting
+  void onTransactionAvailable(int surfaceId) {
+    FabricUIManagerBinding binding = mBinding;
+    if (binding == null) {
+      return;
+    }
+    mMountItemDispatcher.addMountItem(new PullTransactionMountItem(surfaceId, binding));
+    if (UiThreadUtil.isOnUiThread()) {
+      mMountItemDispatcher.tryDispatchMountItems();
+    }
+  }
+
   @SuppressWarnings("unused")
   @AnyThread
   @ThreadConfined(ANY)

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManagerBinding.kt

@@ -85,6 +85,8 @@ internal class FabricUIManagerBinding : HybridClassBase() {
 
   external fun reportMount(surfaceId: Int)
 
+  external fun pullAndExecuteTransaction(surfaceId: Int)
+
   external fun mergeReactRevision(surfaceId: Int)
 
   fun register(

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/mountitems/PullTransactionMountItem.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.fabric.mounting.mountitems
+
+import com.facebook.proguard.annotations.DoNotStripAny
+import com.facebook.react.fabric.FabricUIManagerBinding
+import com.facebook.react.fabric.mounting.MountingManager
+
+/**
+ * Pull model mount item. Enqueued (on the commit thread) when C++ notifies that a transaction is
+ * available for a surface. When it executes on the UI thread it asks C++ to pull the surface's
+ * pending transaction and apply it synchronously, so the diff + batch construction happens on the
+ * UI thread instead of the commit thread (matching iOS).
+ */
+@DoNotStripAny
+internal class PullTransactionMountItem(
+    private val surfaceId: Int,
+    private val binding: FabricUIManagerBinding,
+) : MountItem {
+
+  override fun execute(mountingManager: MountingManager) {
+    binding.pullAndExecuteTransaction(surfaceId)
+  }
+
+  override fun getSurfaceId(): Int = surfaceId
+
+  override fun toString(): String = "PullTransactionMountItem [surfaceId: $surfaceId]"
+}