fix: send empty JSON body to voice session endpoint (fixes 422)

FastAPI 的 create_session 端点声明了 Pydantic request body
(CreateSessionRequest)，虽然所有字段都有默认值，但 FastAPI
仍要求请求包含有效 JSON body（至少 {}）。Flutter 端 dio.post
未传 data 参数导致 Content-Type 缺失，FastAPI 返回 422
Unprocessable Entity。修复：添加 data: {} 发送空 JSON 对象。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

it0_app/lib/features/agent_call/presentation/pages/agent_call_page.dart

@@ -86,7 +86,7 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
     try {
       final dio = ref.read(dioClientProvider);
       final config = ref.read(appConfigProvider);
-      final response = await dio.post('${ApiEndpoints.voice}/sessions');
+      final response = await dio.post('${ApiEndpoints.voice}/sessions', data: {});
       final data = response.data as Map<String, dynamic>;
 
       _sessionId = data['session_id'] as String? ?? data['sessionId'] as String? ?? data['id'] as String?;