로그인 챌린지 루프 방지 가드 추가

userfront/test/login_challenge_loop_guard_test.dart

@@ -0,0 +1,33 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:userfront/core/services/login_challenge_loop_guard.dart';
+
+void main() {
+  group('login_challenge_loop_guard', () {
+    test('mark 이후 cooldown 내 재시도는 차단되고 clear 후 허용된다', () {
+      const challenge = 'loop-guard-test-challenge';
+      loginChallengeLoopGuard.clear(challenge);
+
+      expect(
+        loginChallengeLoopGuard.shouldAllowAutoAccept(challenge),
+        isTrue,
+      );
+
+      loginChallengeLoopGuard.markAutoAcceptAttempt(challenge);
+
+      expect(
+        loginChallengeLoopGuard.shouldAllowAutoAccept(
+          challenge,
+          cooldownMs: 60000,
+        ),
+        isFalse,
+      );
+
+      loginChallengeLoopGuard.clear(challenge);
+
+      expect(
+        loginChallengeLoopGuard.shouldAllowAutoAccept(challenge),
+        isTrue,
+      );
+    });
+  });
+}