feat(exception): GlobalExceptionHandler 新增 WechatApiException handler

解决问题：
- WechatApiException 抛出后未统一处理，会冒泡到 Spring 默认错误页（500 HTML）
- 前端拿不到结构化错误信息

改动：
- 新增 @ExceptionHandler(WechatApiException.class) 方法
- code=400 -> Result.badRequest()（微信业务错误）
- code=502 -> Result.error(502, message)（上游/网络错误）
- 其他(code=500) -> Result.error(message)（内部 JSON 解析错误）
- 日志记录 code 和 message（已确保不含敏感信息）
- 风格与现有 BusinessException handler 保持一致

backend-single/src/test/java/com/emotion/exception/GlobalExceptionHandlerWechatTest.java

@@ -0,0 +1,72 @@
+package com.emotion.exception;
+
+import com.emotion.common.Result;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.mock.web.MockHttpServletRequest;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * GlobalExceptionHandler 的 WechatApiException handler 单元测试
+ *
+ * 验证：
+ * 1. 400 业务错误 → "微信授权失败，请重试"
+ * 2. 500 内部错误 → "微信登录失败，请重试"
+ * 3. 502 上游错误 → "微信服务暂不可用，请稍后重试"
+ */
+@ExtendWith(MockitoExtension.class)
+class GlobalExceptionHandlerWechatTest {
+
+    @InjectMocks
+    private GlobalExceptionHandler handler;
+
+    private MockHttpServletRequest request;
+
+    @BeforeEach
+    void setUp() {
+        request = new MockHttpServletRequest();
+        request.setMethod("POST");
+        request.setRequestURI("/api/wechat/login");
+    }
+
+    @Test
+    void handleWechatApiException_Code400_ReturnsBadRequestWithFriendlyMessage() {
+        WechatApiException ex = new WechatApiException(400, "errcode=40029, errmsg=invalid code (internal detail)", "");
+
+        Result<?> result = handler.handleWechatApiException(ex, request);
+
+        assertNotNull(result);
+        assertEquals("微信授权失败，请重试", result.getMessage());
+        // Ensure internal detail is NOT exposed to frontend
+        assertFalse(result.getMessage().contains("errcode"));
+        assertFalse(result.getMessage().contains("40029"));
+    }
+
+    @Test
+    void handleWechatApiException_Code500_ReturnsInternalErrorWithFriendlyMessage() {
+        WechatApiException ex = new WechatApiException(500, "JSON parse failure at line 1 (internal detail)", "");
+
+        Result<?> result = handler.handleWechatApiException(ex, request);
+
+        assertNotNull(result);
+        assertEquals("微信登录失败，请重试", result.getMessage());
+        assertFalse(result.getMessage().contains("JSON"));
+        assertFalse(result.getMessage().contains("parse"));
+    }
+
+    @Test
+    void handleWechatApiException_Code502_ReturnsBadGatewayWithFriendlyMessage() {
+        WechatApiException ex = new WechatApiException(502, "Connection refused: api.weixin.qq.com (internal detail)", "");
+
+        Result<?> result = handler.handleWechatApiException(ex, request);
+
+        assertNotNull(result);
+        assertEquals("微信服务暂不可用，请稍后重试", result.getMessage());
+        assertFalse(result.getMessage().contains("Connection"));
+        assertFalse(result.getMessage().contains("refused"));
+    }
+}