fix(ast): quote string literals in diagnostics

Preserve source delimiters when rendering string-literal AST nodes so literal text cannot be confused with missing-expression markers.

Author: thiremani

ast/ast.go

@@ -278,7 +278,7 @@ type StringLiteral struct {
 
 func (sl *StringLiteral) expressionNode()  {}
 func (sl *StringLiteral) Tok() token.Token { return sl.Token }
-func (sl *StringLiteral) String() string   { return sl.Token.Literal }
+func (sl *StringLiteral) String() string   { return `"` + sl.Token.Literal + `"` }
 
 // RangeLiteral represents start:stop[:step]
 type RangeLiteral struct {

parser/codeparser_test.go

@@ -401,9 +401,7 @@ func TestParseStructDefinition(t *testing.T) {
 	_, constExists := code.ConstNames["p"]
 	require.True(t, constExists)
 
-	require.Equal(t, `p = Person
-    :name age height
-    Tejas 35 184.5`, stmt.String())
+	require.Equal(t, "p = Person\n    :name age height\n    \"Tejas\" 35 184.5", stmt.String())
 }
 
 func TestStructDefErrors(t *testing.T) {

parser/scriptparser_test.go

@@ -197,6 +197,7 @@ func TestStringLiteral(t *testing.T) {
 	lit, ok := printStmt.Expression.Arguments[0].(*ast.StringLiteral)
 	require.Truef(t, ok, "expected *ast.StringLiteral, got %T", printStmt.Expression.Arguments[0])
 	require.Equal(t, "hello", lit.Token.Literal)
+	require.Equal(t, input, lit.String())
 }
 
 func TestParsingPrefixExpressions(t *testing.T) {