Accept keywords as index type names in ALTER ADD INDEX

The parser was only accepting identifiers (token.IDENT) for index type
names like "set" in "ADD INDEX idx c TYPE set(0)". However, "set" is
tokenized as a keyword (token.SET). This fix allows keywords to be used
as index type names and AFTER index names, matching ClickHouse behavior.

Fixes tests:
- 01932_alter_index_with_order (stmt5, stmt6)
- 03629_storage_s3_disallow_index_alter (stmt3)
- 02131_skip_index_not_materialized (stmt4)

Author: claude

parser/parser.go

@@ -5305,9 +5305,10 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 			if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "TYPE" {
 				p.nextToken()
 				// Type is a function call like bloom_filter(0.025) or vector_similarity('hnsw', 'L2Distance', 1)
+				// Note: Index types can be keywords (e.g., SET) so we accept both IDENT and keywords
 				pos := p.current.Pos
 				typeName := ""
-				if p.currentIs(token.IDENT) {
+				if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
 					typeName = p.current.Value
 					cmd.IndexType = typeName
 					p.nextToken()
@@ -5339,7 +5340,8 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 			// Parse AFTER
 			if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "AFTER" {
 				p.nextToken()
-				if p.currentIs(token.IDENT) {
+				// Index name can be an identifier or keyword
+				if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
 					cmd.AfterIndex = p.current.Value
 					p.nextToken()
 				}

parser/testdata/01932_alter_index_with_order/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt5": true,
-    "stmt6": true
-  }
-}
+{}

parser/testdata/02131_skip_index_not_materialized/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt4": true
-  }
-}
+{}

parser/testdata/03629_storage_s3_disallow_index_alter/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt3": true
-  }
-}
+{}