fix(android): register incoming-call handler on every engine messenger

bindChannelIfUnbound early-returned on later engines, so a FlutterFire
background isolate's messenger never got setMethodCallHandler, breaking
Dart->Native calls (MissingPluginException) from an FCM background handler.
Always register the incoming handler per engine while keeping the outgoing
channel pinned to the first engine (preserves the #1138 Native->Dart fix).

Also fix the .gitignore comment to reference examples/demo_fm.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: fadi-george

.gitignore

@@ -32,7 +32,7 @@ dlcov.log
 **/workspace-state.json
 
 # Per-developer Firebase config for issue #1138 repro scaffolding
-# (only needed because firebase_messaging is wired into examples/demo;
+# (only needed because firebase_messaging is wired into examples/demo_fm;
 # OneSignal itself does NOT need this file).
 **/google-services.json
 **/GoogleService-Info.plist

android/src/main/java/com/onesignal/flutter/FlutterMessengerResponder.java

@@ -14,20 +14,26 @@ abstract class FlutterMessengerResponder {
     BinaryMessenger messenger;
 
     /**
-     * #1138: bind the shared channel only on the first engine. These responders are
-     * process-global singletons but {@code registerWith} runs once per Flutter
-     * engine; FlutterFire's headless background engine would otherwise rebind the
-     * channel to an isolate with no listeners and drop native callbacks.
+     * #1138: bind the outgoing shared channel only on the first engine. These
+     * responders are process-global singletons but {@code registerWith} runs once
+     * per Flutter engine; FlutterFire's headless background engine would otherwise
+     * rebind the channel to an isolate with no listeners and drop native callbacks.
+     *
+     * <p>The incoming-call handler is still registered on every engine's messenger
+     * so Dart->Native calls work from any isolate (e.g. an FCM background handler),
+     * matching the pre-#1138 behavior; only the outgoing Native->Dart channel stays
+     * pinned to the first engine.
      *
      * @return true if this call performed the initial bind.
      */
     boolean bindChannelIfUnbound(BinaryMessenger messenger, String channelName, MethodCallHandler handler) {
+        MethodChannel channel = new MethodChannel(messenger, channelName);
+        channel.setMethodCallHandler(handler);
         if (this.channel != null) {
             return false;
         }
         this.messenger = messenger;
-        this.channel = new MethodChannel(messenger, channelName);
-        this.channel.setMethodCallHandler(handler);
+        this.channel = channel;
         return true;
     }