Fix ALTER ADD INDEX tuple expression parsing

Simplified the index expression parsing in ALTER ADD INDEX to let
parseExpression handle parentheses naturally. This allows tuple
expressions like (a, b, c) to be parsed correctly, matching how
CREATE TABLE INDEX parsing works.

Author: claude

parser/parser.go

@@ -5267,16 +5267,11 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 				Position: p.current.Pos,
 				Name:     idxName,
 			}
-			// Parse expression - can be in parentheses or bare expression until TYPE keyword
-			if p.currentIs(token.LPAREN) {
-				p.nextToken()
+			// Parse expression - let parseExpression handle parentheses naturally
+			// This allows (a, b, c) to be parsed as a tuple
+			if !p.currentIs(token.IDENT) || strings.ToUpper(p.current.Value) != "TYPE" {
 				idx.Expression = p.parseExpression(LOWEST)
 				cmd.IndexExpr = idx.Expression
-				p.expect(token.RPAREN)
-			} else if !p.currentIs(token.IDENT) || strings.ToUpper(p.current.Value) != "TYPE" {
-				// Parse bare expression (not in parentheses) - ends at TYPE keyword
-				idx.Expression = p.parseExpression(ALIAS_PREC)
-				cmd.IndexExpr = idx.Expression
 			}
 			// Parse TYPE
 			if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "TYPE" {

parser/testdata/01114_materialize_clear_index_compact_parts/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt8": true
-  }
-}
+{}

parser/testdata/02710_allow_suspicious_indices/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt14": true,
-    "stmt15": true
-  }
-}
+{}

parser/testdata/02763_mutate_compact_part_with_skip_indices_and_projections/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt5": true
-  }
-}
+{}