feat: implement Linphone SDK integration with video call support

Complete rewrite from stub plugin to (probably) functional liblinphone integration featuring:
- Core SDK bridge with registration, calls, and media controls
- Platform views for remote and local video rendering
- Event channels for registration and call state updates

example/integration_test/plugin_integration_test.dart

@@ -6,7 +6,6 @@
 // For more information about Flutter integration tests, please see
 // https://flutter.dev/to/integration-testing
 
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:integration_test/integration_test.dart';
 
@@ -14,12 +13,4 @@ import 'package:liblinphone_flutter/liblinphone_flutter.dart';
 
 void main() {
   IntegrationTestWidgetsFlutterBinding.ensureInitialized();
-
-  testWidgets('getPlatformVersion test', (WidgetTester tester) async {
-    final LiblinphoneFlutter plugin = LiblinphoneFlutter();
-    final String? version = await plugin.getPlatformVersion();
-    // The version string depends on the host platform running the test, so
-    // just assert that some non-empty string is returned.
-    expect(version?.isNotEmpty, true);
-  });
 }