CodeQL's path-injection flow still traced through _parse_render_request
after the scandir fix -- the tainted find_plugin_dir() result and the
scandir-derived _trusted_plugin_dir() result shared the same variable
name (plugin_dir), reassigned only on the truthy branch. That merge
point apparently isn't treated as a barrier by the flow analysis, so it
kept tracing the pre-reassignment value through to the manifest open().
Split into two distinct names -- candidate_dir (tainted, used only to
call _trusted_plugin_dir) and trusted_dir (the only name used for any
downstream file access) -- so there's no reassigned variable for the
flow to walk through.