test(鉴权): 兜底过滤器用例改用不存在的样例豁免前缀

exemptPrefixes 是配置化操作出口（aiimage.security.admin-guard-exempt-prefixes，
默认空、生产未配置任何值），原用例拿已下线的 /api/admin/shop-credential-checks
当样例路径，容易被误读成「该端点已配好豁免」——那条路径是 2026-09-04 引入
豁免机制时随手取的，并非真的配过。换成显然不存在的 /api/admin/example-exempt，
并注明不要改成真实业务端点。

backend-java/src/test/java/com/nanri/aiimage/config/AdminApiGuardFilterTest.java

@@ -142,10 +142,13 @@ class AdminApiGuardFilterTest {
 
     @Test
     void exemptPrefixPassesThrough() throws Exception {
+        // 豁免前缀是配置项 aiimage.security.admin-guard-exempt-prefixes 留的操作出口，默认空、
+        // 生产未配置任何值。故此处取一个显然不存在的样例前缀——不要换成某个真实业务端点，
+        // 否则会被误读成"该端点已配好豁免"（此前这里是已下线的 shop-credential-checks）。
         AdminAuthSupport authSupport = mock(AdminAuthSupport.class);
-        AdminApiGuardFilter filter = newFilter(authSupport, true, "/api/admin/shop-credential-checks");
+        AdminApiGuardFilter filter = newFilter(authSupport, true, "/api/admin/example-exempt");
         MockHttpServletRequest request =
-                new MockHttpServletRequest("GET", "/api/admin/shop-credential-checks/poll");
+                new MockHttpServletRequest("GET", "/api/admin/example-exempt/poll");
         MockHttpServletResponse response = new MockHttpServletResponse();
         MockFilterChain chain = new MockFilterChain();