Prevent user and system variables in column default and generated values

This change adds validation to prevent user variables (@variable) and system
variables (@@variable) from being used in column default value expressions
and generated column expressions, matching MySQL's behavior.

Added ErrColumnDefaultUserVariable error and validation logic in
validateColumnDefault function to detect UserVar and SystemVar expressions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

sql/analyzer/resolve_column_defaults.go

@@ -233,6 +233,9 @@ func validateColumnDefault(ctx *sql.Context, col *sql.Column, colDefault *sql.Co
 	var err error
 	sql.Inspect(colDefault.Expr, func(e sql.Expression) bool {
 		switch e.(type) {
+		case *expression.UserVar, *expression.SystemVar:
+			err = sql.ErrColumnDefaultUserVariable.New(col.Name)
+			return false
 		case sql.FunctionExpression, *expression.UnresolvedFunction:
 			var funcName string
 			switch expr := e.(type) {

sql/errors.go

@@ -147,6 +147,9 @@ var (
 	// ErrInvalidColumnDefaultValue is returned when column default function value is not wrapped in parentheses for column types excluding datetime and timestamp
 	ErrInvalidColumnDefaultValue = errors.NewKind("Invalid default value for '%s'")
 
+	// ErrColumnDefaultUserVariable is returned when a column default expression contains user or system variables
+	ErrColumnDefaultUserVariable = errors.NewKind("Default value expression of column '%s' cannot refer user or system variables.")
+
 	// ErrInvalidDefaultValueOrder is returned when a default value references a column that comes after it and contains a default expression.
 	ErrInvalidDefaultValueOrder = errors.NewKind(`default value of column "%s" cannot refer to a column defined after it if those columns have an expression default value`)