feat(messaging): remove deprecated functions (#17563)

This PR removes the deprecated `sendMessage` method from the firebase_messaging package as part of preparing for a breaking change release. The method was deprecated due to Firebase decommissioning this functionality in June 2024.

## Changes

- Removed `sendMessage` method from main FirebaseMessaging class
- Removed method from platform interface
- Removed method from method channel implementation  
- Updated tests to remove sendMessage test cases

## Migration Guide

The `sendMessage` method was Android-only and allowed sending FCM messages from the client. To migrate, use Firebase Cloud Functions or your backend server to send messages instead of the client-side method.

## Breaking Change

This removes the deprecated `sendMessage` method entirely. Apps using this method will need to migrate to server-side message sending before upgrading.

Author: Lyokone

packages/firebase_messaging/firebase_messaging/lib/src/messaging.dart

@@ -212,31 +212,6 @@ class FirebaseMessaging extends FirebasePluginPlatform {
     );
   }
 
-  /// Send a new [RemoteMessage] to the FCM server. Android only.
-  /// Firebase will decommission in June 2024: https://firebase.google.com/docs/reference/android/com/google/firebase/messaging/FirebaseMessaging#send
-  @Deprecated(
-      'This will be removed in a future release. Firebase will decommission in June 2024')
-  Future<void> sendMessage({
-    String? to,
-    Map<String, String>? data,
-    String? collapseKey,
-    String? messageId,
-    String? messageType,
-    int? ttl,
-  }) {
-    if (ttl != null) {
-      assert(ttl >= 0);
-    }
-    return _delegate.sendMessage(
-      to: to ?? '${app.options.messagingSenderId}@fcm.googleapis.com',
-      data: data,
-      collapseKey: collapseKey,
-      messageId: messageId,
-      messageType: messageType,
-      ttl: ttl,
-    );
-  }
-
   /// Enable or disable auto-initialization of Firebase Cloud Messaging.
   Future<void> setAutoInitEnabled(bool enabled) async {
     return _delegate.setAutoInitEnabled(enabled);

packages/firebase_messaging/firebase_messaging_platform_interface/lib/src/method_channel/method_channel_messaging.dart

@@ -358,35 +358,6 @@ class MethodChannelFirebaseMessaging extends FirebaseMessagingPlatform {
     }
   }
 
-  @override
-  Future<void> sendMessage({
-    required String to,
-    Map<String, String>? data,
-    String? collapseKey,
-    String? messageId,
-    String? messageType,
-    int? ttl,
-  }) async {
-    if (defaultTargetPlatform != TargetPlatform.android) {
-      throw UnimplementedError(
-          'Sending of messages from the Firebase Messaging SDK is only supported on Android devices.');
-    }
-
-    try {
-      await channel.invokeMapMethod('Messaging#sendMessage', {
-        'appName': app.name,
-        'to': to,
-        'data': data,
-        'collapseKey': collapseKey,
-        'messageId': messageId,
-        'messageType': messageType,
-        'ttl': ttl,
-      });
-    } catch (e, stack) {
-      convertPlatformException(e, stack);
-    }
-  }
-
   @override
   Future<void> subscribeToTopic(String topic) async {
     await _APNSTokenCheck();

packages/firebase_messaging/firebase_messaging_platform_interface/lib/src/platform_interface/platform_interface_messaging.dart

@@ -295,18 +295,6 @@ abstract class FirebaseMessagingPlatform extends PlatformInterface {
         'setForegroundNotificationPresentationOptions() is not implemented');
   }
 
-  /// Send a new [RemoteMessage] to the FCM server.
-  Future<void> sendMessage({
-    required String to,
-    Map<String, String>? data,
-    String? collapseKey,
-    String? messageId,
-    String? messageType,
-    int? ttl,
-  }) {
-    throw UnimplementedError('sendMessage() is not implemented');
-  }
-
   /// Subscribe to topic in background.
   ///
   /// [topic] must match the following regular expression:

packages/firebase_messaging/firebase_messaging_platform_interface/test/method_channel_tests/method_channel_messaging_test.dart

@@ -3,12 +3,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:firebase_messaging_platform_interface/src/method_channel/method_channel_messaging.dart';
+import 'package:firebase_core/firebase_core.dart';
 import 'package:firebase_messaging_platform_interface/firebase_messaging_platform_interface.dart';
+import 'package:firebase_messaging_platform_interface/src/method_channel/method_channel_messaging.dart';
 import 'package:flutter/foundation.dart';
-import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/services.dart';
-import 'package:firebase_core/firebase_core.dart';
+import 'package:flutter_test/flutter_test.dart';
 import 'package:mockito/mockito.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
@@ -29,7 +29,6 @@ void main() {
         log.add(call);
         switch (call.method) {
           case 'Messaging#deleteToken':
-          case 'Messaging#sendMessage':
           case 'Messaging#subscribeToTopic':
           case 'Messaging#unsubscribeFromTopic':
             return null;