refactor: replace gperr with standard errors package and simplify string parsing

- Replace gperr.Error return types with standard error across test files
- Replace gperr.New with errors.New in validation and serialization tests
- Update API documentation in README files to use error instead of gperr.Error
- Simplify string parsing using strings.Cut in docker/label.go
- Update benchmarks to use NewTestEntrypoint and remove task package dependency

internal/idlewatcher/provider/README.md

@@ -36,7 +36,7 @@ type Provider interface {
 
     // Status and monitoring
     ContainerStatus(ctx context.Context) (ContainerStatus, error)
-    Watch(ctx context.Context) (eventCh <-chan events.Event, errCh <-chan gperr.Error)
+    Watch(ctx context.Context) (eventCh <-chan events.Event, errCh <-chan error)
 
     // Cleanup
     Close()