chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/publicip/api/echoip.go

@@ -70,11 +70,9 @@ func (e *echoip) FetchInfo(ctx context.Context, ip netip.Addr) (
 	switch response.StatusCode {
 	case http.StatusOK:
 	case http.StatusTooManyRequests:
-		return result, fmt.Errorf("%w from %s: %s",
-			ErrTooManyRequests, url, response.Status)
+		return result, fmt.Errorf("%w from %s: %s", ErrTooManyRequests, url, response.Status)
 	default:
-		return result, fmt.Errorf("%w from %s: %s",
-			ErrBadHTTPStatus, url, response.Status)
+		return result, fmt.Errorf("bad HTTP status received from %s: %s", url, response.Status)
 	}
 
 	decoder := json.NewDecoder(response.Body)