fix: Add missing TestError.skip case to fix Mac CI test failures

The NgrokServiceTests were failing in CI with "cannot find 'TestError' in scope"
because TestError.skip was being used but the skip case was not defined.

- Add TestError.skip(String) case to TestFixtures
- Update NgrokServiceTests to use TestFixtures.TestError.skip for proper namespacing

mac/VibeTunnelTests/NgrokServiceTests.swift

@@ -360,7 +360,7 @@ struct NgrokServiceTests {
 
         // Check if we have an auth token
         guard service.hasAuthToken else {
-            throw XCTSkip("Ngrok auth token not configured")
+            throw TestFixtures.TestError.skip("Ngrok auth token not configured")
         }
 
         // This would require actual ngrok installation

mac/VibeTunnelTests/Utilities/TestFixtures.swift

@@ -131,5 +131,6 @@ extension TestFixtures {
 
     enum TestError: Error {
         case timeout
+        case skip(String)
     }
 }