Fix gh extension list detection to match github/gh-aw owner-prefix format (#33168)

The grep regex `(^|[[:space:]])gh-aw($|[[:space:]])` failed to match
`github/gh-aw` (the format that modern `gh extension list` returns),
because `gh-aw` was preceded by `/` rather than whitespace or line
start. This caused the guard check to always evaluate false, so the
install step would always attempt `gh extension install github/gh-aw`
even when the extension was already present, resulting in:

  there is already an installed extension that provides the "aw" command

Fix: add `|/` to the leading-character alternatives so the pattern also
matches the `owner/gh-aw` output format:

  (^|[[:space:]]|/)gh-aw($|[[:space:]]|$)

Also update the corresponding test assertion and golden file, and
recompile all lock files to propagate the change.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>

Author: Copilot