Fix SELECT item alias for wrapped text literals

The alias-inference logic in translate_select_item() treated any item
whose translation matched the inner text literal's translation as a bare
literal and used the literal value as the alias. This misfired for the
CONVERT(expr USING charset) form, where the translator peels the wrapper
and returns the inner literal unchanged — e.g.
"SELECT CONVERT('Customer' USING utf8mb4)" produced alias "Customer"
instead of the original expression text.

Replace the string-equality heuristic with a structural walk: descend
the AST until we reach a textLiteral, bailing out at any level that
doesn't have exactly one child node. Operator expressions, CAST,
CONVERT, BINARY, unary minus, and function calls all introduce a second
child at some level and are correctly rejected.

Author: JanJakes

packages/mysql-on-sqlite/src/sqlite/class-wp-pdo-mysql-on-sqlite.php

@@ -4696,11 +4696,20 @@ public function translate_select_item( WP_Parser_Node $node ): string {
 		 *
 		 * For example, for "SELECT 'abc'", the resulting column name is "abc"
 		 * in MySQL, but would be "'abc'" in SQLite if an alias was not used.
+		 *
+		 * Descend the AST until we reach a textStringLiteral. If at any level
+		 * we don't have a single child node, bail out; it's not a bare literal.
 		 */
-		$text_string_literal    = $node->get_first_descendant_node( 'textStringLiteral' );
-		$is_text_string_literal = $text_string_literal && $item === $this->translate( $text_string_literal );
-		if ( $is_text_string_literal ) {
-			$alias = $text_string_literal->get_first_child_token()->get_value();
+		$current = $node;
+		while ( 'textStringLiteral' !== $current->rule_name ) {
+			$children = $current->get_children();
+			if ( 1 !== count( $children ) || ! $children[0] instanceof WP_Parser_Node ) {
+				break;
+			}
+			$current = $children[0];
+		}
+		if ( 'textStringLiteral' === $current->rule_name ) {
+			$alias = $current->get_first_child_token()->get_value();
 
 			// When the literal value contains a NULL byte, MySQL truncates the
 			// resulting identifier at the position of the first one of them.

packages/mysql-on-sqlite/tests/WP_SQLite_Driver_Translation_Tests.php

@@ -120,12 +120,12 @@ public function testConvert(): void {
 
 		// CONVERT(expr USING charset) → expr
 		$this->assertQuery(
-			"SELECT 'Customer' AS `Customer`",
+			"SELECT 'Customer' AS `CONVERT('Customer' USING utf8mb4)`",
 			"SELECT CONVERT('Customer' USING utf8mb4)"
 		);
 
 		$this->assertQuery(
-			"SELECT 'test' AS `test`",
+			"SELECT 'test' AS `CONVERT('test' USING utf8)`",
 			"SELECT CONVERT('test' USING utf8)"
 		);