refactor(ui): improve message input controller

packages/stream_chat_flutter/example/lib/tutorial_part_4.dart

@@ -126,7 +126,9 @@ class ThreadPage extends StatelessWidget {
             ),
           ),
           MessageInput(
-            parentMessage: parent,
+            messageInputController: MessageInputController(
+              message: Message(parentId: parent!.id),
+            ),
           ),
         ],
       ),

packages/stream_chat_flutter/example/lib/tutorial_part_6.dart

@@ -166,7 +166,9 @@ class ThreadPage extends StatelessWidget {
             ),
           ),
           MessageInput(
-            parentMessage: parent,
+            messageInputController: MessageInputController(
+              message: Message(parentId: parent!.id),
+            ),
           ),
         ],
       ),