Fix literal \", nil\" in handleStagingRemove error format string

The format string in handleStagingRemove mistakenly included ", nil"
as literal text instead of keeping it outside the Sprintf call,
causing every HTTP failure error message to end with ", nil".

Adds a regression test using httptest to verify the error message
is clean on a simulated 403 response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hussaintauseef

connection.go

@@ -587,7 +587,7 @@ func (c *conn) handleStagingRemove(ctx context.Context, presignedUrl string, hea
 	content, err := io.ReadAll(res.Body)
 
 	if err != nil || !Succeeded(res) {
-		return dbsqlerrint.NewDriverError(ctx, fmt.Sprintf("staging operation over HTTP was unsuccessful: %d-%s, nil", res.StatusCode, content), nil)
+		return dbsqlerrint.NewDriverError(ctx, fmt.Sprintf("staging operation over HTTP was unsuccessful: %d-%s", res.StatusCode, content), nil)
 	}
 
 	return nil

staging_test.go

@@ -1,11 +1,29 @@
 package dbsql
 
 import (
+	"context"
+	"net/http"
+	"net/http/httptest"
 	"testing"
 
+	"github.com/databricks/databricks-sql-go/internal/config"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
+func TestHandleStagingRemoveHTTPError(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusForbidden)
+		w.Write([]byte("Access Denied"))
+	}))
+	defer server.Close()
+
+	c := &conn{cfg: config.WithDefaults()}
+	err := c.handleStagingRemove(context.Background(), server.URL, map[string]string{})
+	require.NotNil(t, err)
+	assert.NotContains(t, err.Error(), ", nil", "error message should not contain literal \", nil\"")
+}
+
 func TestPathAllowed(t *testing.T) {
 	t.Run("Should not allow paths that don't share directory", func(t *testing.T) {
 		stagingAllowedLocalPath := []string{"/var/www/html"}