Clicking a folder path in an assistant message opened a file tab that
failed with "Requested path is not a file". Extension-less
workspace-relative paths without a line number are now probed with a
directory listing before opening: directories open the Files explorer
navigated to that folder, everything else opens as a file like before.
Lexical directories (trailing slash, workspace root) now navigate the
explorer to the folder instead of only preloading its listing.
