aiohttp's BasicAuth.decode() raises ValueError for any non-Basic auth
method (e.g. Bearer tokens). This propagated as an unhandled exception,
causing a 500 response instead of the expected 401 Unauthorized.
Catch the ValueError in _process_basic() and raise HTTPUnauthorized with
the WWW-Authenticate realm header so clients get a proper 401 response.
Fixes SUPERVISOR-BFG
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>