Closes #38620

`Url::from_file_path` and `Url::from_directory_path` assume the path
style of the target they were compiled for, so we can't use them in
general. So, switch from `file://` to encoding the absolute path (for
mentions that have one) as a query parameter, which works no matter the
platforms. We'll still parse the old `file://` mention URIs for
compatibility with thread history.

Release Notes:

- windows: Fixed a crash when using `@mentions` in agent threads when
remoting from Windows to Linux or WSL.