Add IF EXISTS support for RENAME COLUMN in ALTER TABLE

RENAME COLUMN IF EXISTS was not being parsed correctly - the IF token
was being treated as the column name. This fix adds proper IF EXISTS
handling for RENAME COLUMN, similar to DROP COLUMN.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

parser/parser.go

@@ -5899,6 +5899,12 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 		if p.currentIs(token.COLUMN) {
 			cmd.Type = ast.AlterRenameColumn
 			p.nextToken()
+			// Handle IF EXISTS
+			if p.currentIs(token.IF) {
+				p.nextToken()
+				p.expect(token.EXISTS)
+				cmd.IfExists = true
+			}
 			// Parse column name (can be dotted like n.x for nested columns)
 			if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
 				cmd.ColumnName = p.parseDottedIdentifier()

parser/testdata/03595_alter_if_exists_mixed_commands/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt7": true
-  }
-}
+{}

parser/testdata/03595_alter_if_exists_runtime_check/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt11": true
-  }
-}
+{}