feat: 原生支持 /v1/responses 接口与 Chat Completions 双协议自适应，增加端到端联调测试

kouyu_english/test/ai_config_test.dart

@@ -6,7 +6,7 @@ import 'package:kouyu_english/core/models.dart';
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
-  group('AiService resolveEndpointUri (/v1/chat/completions)', () {
+  group('AiService resolveEndpointUri', () {
     test('resolves OpenAI endpoint ending in /v1 to /v1/chat/completions', () {
       final uri = AiService.resolveEndpointUri(
         provider: AiProviderType.openAi,
@@ -43,13 +43,13 @@ void main() {
       expect(uri.toString(), equals('https://codex.slcydia.fun/v1/chat/completions'));
     });
 
-    test('converts /responses endpoint to /chat/completions', () {
+    test('preserves /responses endpoint for OpenAI Responses API support', () {
       final uri = AiService.resolveEndpointUri(
         provider: AiProviderType.compatible,
         endpoint: 'https://codex.slcydia.fun/v1/responses',
         model: 'gpt-4o-mini',
       );
-      expect(uri.toString(), equals('https://codex.slcydia.fun/v1/chat/completions'));
+      expect(uri.toString(), equals('https://codex.slcydia.fun/v1/responses'));
     });
 
     test('resolves Gemini endpoint to :generateContent', () {