test(middleware): add unit tests for middleware behavior

- remove redundant check on registry.Update and check if slug exist before locking the mutex
- Update SonarQube action to not use Go cache when setting up Go

.gitea/workflows/sonarqube.yml

@@ -18,6 +18,7 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version: '1.25.5'
+          cache: false
 
       - name: Install dependencies
         run: go mod tidy