fix: 修复腾讯混元API签名算法，使用UTC时区和正确的Content-Type

src/main/java/xuan/service/HunYuanAiService.java

@@ -31,6 +31,9 @@ public class HunYuanAiService {
             .build();
 
     public String chat(String systemPrompt, String userMessage) {
+        if (SECRET_ID == null || SECRET_KEY == null) {
+            return "AI服务未配置，请设置TENCENT_SECRET_ID和TENCENT_SECRET_KEY环境变量。";
+        }
         try {
             String response = sendRequest(systemPrompt, userMessage);
             return parseResponse(response);
@@ -41,8 +44,11 @@ public class HunYuanAiService {
     }
 
     private String sendRequest(String systemPrompt, String userMessage) throws IOException {
-        String timestamp = String.valueOf(System.currentTimeMillis() / 1000);
-        String date = new SimpleDateFormat("yyyy-MM-dd").format(new Date(Long.parseLong(timestamp) * 1000));
+        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
+        sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
+        Date now = new Date();
+        String date = sdf.format(now);
+        String timestamp = String.valueOf(now.getTime() / 1000);
         
         JSONObject requestBody = new JSONObject();
         requestBody.put("Model", "hunyuan-lite");
@@ -65,22 +71,13 @@ public class HunYuanAiService {
         
         String payload = requestBody.toJSONString();
         
-        String credential = generateCredential(timestamp, date, payload);
-        
-        JSONObject headers = new JSONObject();
-        headers.put("Authorization", credential);
-        headers.put("Content-Type", "application/json");
-        headers.put("Host", HOST);
-        headers.put("X-TC-Action", ACTION);
-        headers.put("X-TC-Version", VERSION);
-        headers.put("X-TC-Timestamp", timestamp);
-        headers.put("X-TC-Region", REGION);
+        String authorization = generateAuthorization(timestamp, date, payload);
         
         Request request = new Request.Builder()
                 .url("https://" + HOST)
-                .post(RequestBody.create(payload, MediaType.parse("application/json")))
-                .addHeader("Authorization", credential)
-                .addHeader("Content-Type", "application/json")
+                .post(RequestBody.create(payload, MediaType.parse("application/json; charset=utf-8")))
+                .addHeader("Authorization", authorization)
+                .addHeader("Content-Type", "application/json; charset=utf-8")
                 .addHeader("Host", HOST)
                 .addHeader("X-TC-Action", ACTION)
                 .addHeader("X-TC-Version", VERSION)
@@ -89,19 +86,20 @@ public class HunYuanAiService {
                 .build();
         
         try (Response response = client.newCall(request).execute()) {
+            String responseBody = response.body() != null ? response.body().string() : "";
             if (!response.isSuccessful()) {
-                throw new IOException("Unexpected response code: " + response.code());
+                return "{\"Response\":{\"Error\":{\"Message\":\"HTTP " + response.code() + ": " + responseBody + "\"}}}";
             }
-            return response.body().string();
+            return responseBody;
         }
     }
 
-    private String generateCredential(String timestamp, String date, String payload) {
+    private String generateAuthorization(String timestamp, String date, String payload) {
         try {
             String httpRequestMethod = "POST";
             String canonicalUri = "/";
             String canonicalQueryString = "";
-            String canonicalHeaders = "content-type:application/json\nhost:" + HOST + "\n";
+            String canonicalHeaders = "content-type:application/json; charset=utf-8\nhost:" + HOST + "\n";
             String signedHeaders = "content-type;host";
             
             String hashedRequestPayload = sha256Hex(payload);
@@ -131,7 +129,7 @@ public class HunYuanAiService {
                     "SignedHeaders=" + signedHeaders + ", " +
                     "Signature=" + signature;
         } catch (Exception e) {
-            throw new RuntimeException("Failed to generate credential", e);
+            throw new RuntimeException("Failed to generate authorization", e);
         }
     }