Fix wrongly format the string in UnaryExpr

[git-hulk]

This closes #122

PR Overview

This PR addresses the formatting of unary expressions in the AST by updating the string representation of a UnaryExpr and adjusting its parsing logic.

• Update UnaryExpr.String() to compose the operator and operand using the token’s string representation.
• Refactor parseUnaryExpr in parser_column.go to rename a local variable and update the conversion for setting the operator in a UnaryExpr.

Reviewed Changes

File	Description
parser/ast.go	Updated the String method for UnaryExpr to use token-based formatting.
parser/parser_column.go	Updated variable naming and conversion for operator determination in unary expressions.

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

parser/ast.go:6648

• Confirm that inserting a space between the operator and operand is desired in all cases, as it may alter the formatting for certain unary operators.

return string(n.Kind) + " " + n.Expr.String()

parser/parser_column.go:319

• Verify that converting op.ToString() to TokenKind reliably produces the correct token type for unary expressions, as discrepancies may occur if the string representation differs from expected token values.

Kind:     TokenKind(op.ToString()),

[Lance726]

LGTM