refactor(exception): 将自定义异常改为运行时异常

- 修改 CustomException 继承自 RuntimeException
- 更新异常处理逻辑以适应运行时异常
- 添加注释说明异常处理机制
- 确保全局异常处理器能正确捕获该异常

src/main/java/work/baiyun/chronicdiseaseapp/exception/CustomException.java

@@ -1,6 +1,10 @@
 package work.baiyun.chronicdiseaseapp.exception;
 
-public class CustomException extends Exception {
-    public CustomException(String message) { super(message);
+/**
+ * 自定义运行时业务异常，交由全局异常处理器处理
+ */
+public class CustomException extends RuntimeException {
+    public CustomException(String message) {
+        super(message);
     }
 }