feat: 调用 DeepSeek 时关闭思考模式

DeepSeek 默认开启思考，思考 token 按输出计费，且可能耗尽较小的
max_tokens 导致 JSON 截断。对 deepseek.com 地址：chat/completions
发送 thinking.type=disabled，Responses 发送 reasoning.effort=none；
其他服务商请求体保持不变。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kouyu_english/lib/core/ai_service.dart

@@ -221,6 +221,11 @@ class AiService {
 
   static bool _isHttpUri(Uri uri) => uri.scheme == 'https' || uri.scheme == 'http';
 
+  static bool _isDeepSeek(Uri uri) {
+    final host = uri.host.toLowerCase();
+    return host == 'deepseek.com' || host.endsWith('.deepseek.com');
+  }
+
   static bool _isSuccess(http.Response response) =>
       response.statusCode >= 200 && response.statusCode < 300;
 
@@ -373,6 +378,27 @@ class AiService {
     String reasoningEffort = 'low',
   }) {
     final isResponses = uri.path.endsWith('/responses');
+    // DeepSeek thinks by default. Every request here is a short JSON answer
+    // with a small token cap, so reasoning only adds cost and can exhaust the
+    // cap before the JSON is written. Other providers may reject these
+    // DeepSeek-specific switches, so they keep the original payload.
+    if (_isDeepSeek(uri)) {
+      return isResponses
+          ? {
+              'model': model,
+              'input': messages,
+              'reasoning': {'effort': 'none'},
+              'temperature': ?temperature,
+              'max_output_tokens': ?maxTokens,
+            }
+          : {
+              'model': model,
+              'messages': messages,
+              'thinking': {'type': 'disabled'},
+              'temperature': ?temperature,
+              'max_tokens': ?maxTokens,
+            };
+    }
     if (isResponses) {
       return {
         'model': model,