ax(mining): remove banned errors import from errors_test.go

Replace errors.New() with ErrInternal()/ErrTimeout() and errors.Unwrap(err)
with err.Unwrap() — the MiningError type provides both without the banned import.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/errors_test.go

@@ -1,7 +1,6 @@
 package mining
 
 import (
-	"errors"
 	"net/http"
 	"testing"
 )
@@ -17,14 +16,14 @@ func TestErrors_Error_Good(t *testing.T) {
 
 // TestErrors_Error_Bad verifies the error string includes the underlying cause
 func TestErrors_Error_Bad(t *testing.T) {
-	cause := errors.New("underlying error")
+	cause := ErrInternal("underlying error")
 	err := NewMiningError(ErrCodeStartFailed, "failed to start").WithCause(cause)
 
 	if err.Cause != cause {
 		t.Error("Cause was not set")
 	}
 
-	if errors.Unwrap(err) != cause {
+	if err.Unwrap() != cause {
 		t.Error("Unwrap did not return cause")
 	}
 }
@@ -136,7 +135,7 @@ func TestErrors_PredefinedErrors_Good(t *testing.T) {
 
 // TestErrors_Chaining_Ugly verifies error chaining survives multiple WithCause/WithDetails/WithSuggestion calls
 func TestErrors_Chaining_Ugly(t *testing.T) {
-	cause := errors.New("network timeout")
+	cause := ErrTimeout("network timeout")
 	err := ErrConnectionFailed("pool:3333").
 		WithCause(cause).
 		WithDetails("timeout after 30s").