chore: remove the TranslatedMessage class and add the i18n field to the Message class

Tests have been updated to reflect the removal of this class

packages/stream_chat/test/src/api/channel_test.dart

@@ -1610,9 +1610,12 @@ void main() {
       const messageId = 'test-message-id';
       const language = 'hi'; // Hindi
       const translatedMessageText = 'नमस्ते';
-      final translatedMessage = TranslatedMessage(const {
-        language: translatedMessageText,
-      });
+
+      final translatedMessage = Message(
+        i18n: const {
+          language: translatedMessageText,
+        },
+      );
 
       when(() => client.translateMessage(messageId, language)).thenAnswer(
         (_) async => TranslateMessageResponse()..message = translatedMessage,

packages/stream_chat/test/src/api/client_test.dart

@@ -2109,9 +2109,11 @@ void main() {
       const messageId = 'test-message-id';
       const language = 'hi'; // Hindi
       const translatedMessageText = 'नमस्ते';
-      final translatedMessage = TranslatedMessage(const {
-        language: translatedMessageText,
-      });
+      final translatedMessage = Message(
+        i18n: const {
+          language: translatedMessageText,
+        },
+      );
 
       when(() => api.message.translateMessage(messageId, language)).thenAnswer(
         (_) async => TranslateMessageResponse()..message = translatedMessage,