test: 补充后端单元测试与覆盖率配置

src/test/java/com/guo/learningprogresstracker/exception/AppExceptionTest.java

@@ -0,0 +1,18 @@
+package com.guo.learningprogresstracker.exception;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * AppException 构造与消息测试。
+ */
+class AppExceptionTest {
+
+    @Test
+    void constructor_shouldKeepMessage() {
+        AppException ex = new AppException("数据库连接失败");
+
+        assertEquals("数据库连接失败", ex.getMessage());
+    }
+}