Add extensive unit tests across the codebase (api/errors, cache, many models tests, util/id_normalizer and others) to improve coverage and validate behavior. Replace ServiceWatcher.isRunning bool with atomic.Bool for safer concurrent access, update Start/Stop logic and related imports, and adjust service_watcher_test accordingly. Also update traefik_fetcher_test to use atomic counters for request counting and add minor import changes (sync/atomic).
