Narrow preserveLeadingComments property to private

It is read only inside BaseMultiQueryParser::buildQuery(); no subclass
accesses it, so `protected` advertised an extension point that does not
exist. Tighten the boundary to `private`.

Co-Authored-By: Claude Code

Author: JanTvrdik

src/BaseMultiQueryParser.php

@@ -18,7 +18,7 @@ abstract class BaseMultiQueryParser implements IMultiQueryParser
 	 *                                       leading whitespace is stripped.
 	 */
 	public function __construct(
-		protected bool $preserveLeadingComments = false,
+		private bool $preserveLeadingComments = false,
 	) {
 	}