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/metrics/uptime/README.md

@@ -303,7 +303,7 @@ curl "http://localhost:8080/api/uptime?period=1d&limit=20&offset=0&keyword=docke
 
 ```javascript
 const ws = new WebSocket(
-  "ws://localhost:8080/api/uptime?period=1m&interval=5s"
+  "ws://localhost:8080/api/uptime?period=1m&interval=5s",
 );
 
 ws.onmessage = (event) => {