fix: replace flaky timing-based test with deterministic assertion

The constant-time comparison test used Date.now() deltas with a 10ms
threshold which is unreliable in CI. Replace with deterministic tests
that verify both same-length and different-length key rejection paths
work correctly. The actual timing-safe behavior is guaranteed by
Node's crypto.timingSafeEqual which the guard uses.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/common/guards/api-key.guard.spec.ts

@@ -113,34 +113,24 @@ describe("ApiKeyGuard", () => {
       const validApiKey = "test-api-key-12345";
       vi.mocked(mockConfigService.get).mockReturnValue(validApiKey);
 
-      const startTime = Date.now();
-      const context1 = createMockExecutionContext({
-        "x-api-key": "wrong-key-short",
+      // Verify that same-length keys are compared properly (exercises timingSafeEqual path)
+      // and different-length keys are rejected before comparison
+      const sameLength = createMockExecutionContext({
+        "x-api-key": "test-api-key-12344", // Same length, one char different
+      });
+      const differentLength = createMockExecutionContext({
+        "x-api-key": "short", // Different length
       });
 
-      try {
-        guard.canActivate(context1);
-      } catch {
-        // Expected to fail
-      }
-      const shortKeyTime = Date.now() - startTime;
+      // Both should throw, proving the comparison logic handles both cases
+      expect(() => guard.canActivate(sameLength)).toThrow("Invalid API key");
+      expect(() => guard.canActivate(differentLength)).toThrow("Invalid API key");
 
-      const startTime2 = Date.now();
-      const context2 = createMockExecutionContext({
-        "x-api-key": "test-api-key-12344", // Very close to correct key
+      // Correct key should pass
+      const correct = createMockExecutionContext({
+        "x-api-key": validApiKey,
       });
-
-      try {
-        guard.canActivate(context2);
-      } catch {
-        // Expected to fail
-      }
-      const longKeyTime = Date.now() - startTime2;
-
-      // Times should be similar (within 10ms) to prevent timing attacks
-      // Note: This is a simplified test; real timing attack prevention
-      // is handled by crypto.timingSafeEqual
-      expect(Math.abs(shortKeyTime - longKeyTime)).toBeLessThan(10);
+      expect(guard.canActivate(correct)).toBe(true);
     });
   });
 });