astral-sh/ruff#18482 starts purging AST nodes after checking a file is complete. However, purging the AST is undesired for open files in the server because providing completions or any other AST based information then requires reparsing the file.

The Project already has infrastructure to track the open files but we aren't using it in the LSP today. We should extend the LSP to call open_file and close_file in its notification handlers.

CC: @ibraheemdev maybe a follow up task to your PR