chore(test): add ability to mock rust calls for tests

lib/features/chat/chat_page.dart

@@ -102,6 +102,7 @@ class ChatPage extends HookWidget {
                       icon: const Icon(Icons.add),
                     ),
                     IconButton.filled(
+                      key: const Key('send_message_button'),
                       onPressed: () {
                         _onSendMessage(
                           messageController.text,

test/features/chat/chat_page_test.dart

@@ -2,16 +2,40 @@ import 'package:flutter/material.dart';
 import 'package:gosling/features/chat/chat_page.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:gosling/generated/rust/frb_generated.dart';
+import 'package:mocktail/mocktail.dart';
 
+import '../../helpers/mocks.dart';
 import '../../helpers/widget_helpers.dart';
 
-void main() {
-  testWidgets('should have logo and input field', (WidgetTester tester) async {
+Future<void> main() async {
+  final mockApi = MockRustLibApi();
+  RustLib.initMock(api: mockApi);
+
+  testWidgets('should have correct initial content',
+      (WidgetTester tester) async {
     await tester.pumpWidget(
       WidgetHelpers.testableWidget(child: const ChatPage()),
     );
 
     expect(find.byType(SvgPicture), findsOneWidget);
     expect(find.byType(TextField), findsOneWidget);
+    expect(find.byKey(const Key('send_message_button')), findsOneWidget);
+  });
+
+  testWidgets('should send message when button is pressed',
+      (WidgetTester tester) async {
+    when(() => mockApi.crateApiSimpleGreet(name: "Hello"))
+        .thenReturn("Hello Test");
+
+    await tester.pumpWidget(
+      WidgetHelpers.testableWidget(child: const ChatPage()),
+    );
+
+    await tester.enterText(find.byType(TextField), 'Hello');
+    await tester.tap(find.byKey(const Key('send_message_button')));
+    await tester.pump();
+
+    expect(find.text('Hello Test'), findsOneWidget);
   });
 }