Handle INTERVAL '2' AS alias unit syntax in parser

Support for aliases on the value expression in INTERVAL syntax,
like INTERVAL '2' AS n minute.

Fixes 01523_interval_operator_support_string_literal (now all 25 pass).

Author: claude

parser/expression.go

@@ -1444,6 +1444,16 @@ func (p *Parser) parseInterval() ast.Expression {
 	// Use ALIAS_PREC to prevent consuming the unit as an alias
 	expr.Value = p.parseExpression(ALIAS_PREC)
 
+	// Handle INTERVAL '2' AS n minute - where AS n is alias on the value
+	if p.currentIs(token.AS) {
+		p.nextToken() // skip AS
+		if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
+			alias := p.current.Value
+			p.nextToken()
+			expr.Value = p.wrapWithAlias(expr.Value, alias)
+		}
+	}
+
 	// Parse unit (interval units are identifiers like DAY, MONTH, etc.)
 	if p.currentIs(token.IDENT) {
 		expr.Unit = strings.ToUpper(p.current.Value)

parser/testdata/01523_interval_operator_support_string_literal/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt19": true
-  }
-}
+{}