Fix streaming and file IO edge cases

Repair streaming callback/error handling and make file/session handling safer so the core API behaves correctly under real usage.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dstalk-core/src/ai/deepseek_api.cpp

@@ -177,9 +177,8 @@ ChatResult DeepSeekClient::chat_stream(
     headers["Authorization"] = "Bearer " + impl_->config.api_key;
 
     ChatResult result;
-    result.ok = true;
 
-    impl_->http.post_stream(host, "443", target_path, body, headers,
+    auto resp = impl_->http.post_stream(host, "443", target_path, body, headers,
         [&](const std::string& line) -> bool {
             if (line.empty()) return true;
             std::string token;
@@ -189,9 +188,36 @@ ChatResult DeepSeekClient::chat_stream(
             return on_token ? on_token(token, userdata) : true;
         });
 
+    result.http_status = resp.status_code;
+
+    // 检查传输层错误或非 2xx 状态
+    if (resp.status_code < 200 || resp.status_code >= 300) {
+        result.ok = false;
+        // 尝试从响应 body 提取错误信息（与 parse_response 等同逻辑）
+        try {
+            auto jv = json::parse(resp.body);
+            auto obj = jv.as_object();
+            if (obj.contains("error")) {
+                auto err = obj["error"].as_object();
+                result.error = json::value_to<std::string>(err["message"]);
+            }
+        } catch (...) {
+        }
+        if (result.error.empty()) {
+            if (resp.status_code <= 0) {
+                result.error = "transport error";
+            } else {
+                result.error = "HTTP " + std::to_string(resp.status_code);
+            }
+        }
+        return result;
+    }
+
     if (result.content.empty()) {
         result.ok = false;
         result.error = "no content received";
+    } else {
+        result.ok = true;
     }
     return result;
 }