fix: ensure URI path starts with slash when splitting terminal with authority (fixes #316035) (#316040)

fix: ensure path starts with slash when splitting terminal with authority

When splitting a terminal whose parent has a remote authority, the child's
cwd string (e.g. a Windows path like C:\Users\foo) was passed directly as
the URI path component. URI validation requires paths to start with '/'
when an authority is present. Prepend '/' when needed.

Fixes #316035

Co-authored-by: vs-code-engineering[bot] <122617954+vs-code-engineering[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>