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/watcher/README.md

@@ -40,7 +40,7 @@ Alias to `events.Event` for convenience.
 type Watcher interface {
     // Events returns channels for receiving events and errors.
     // The channels are closed when the context is cancelled.
-    Events(ctx context.Context) (<-chan Event, <-chan gperr.Error)
+    Events(ctx context.Context) (<-chan Event, <-chan error)
 }
 ```