POP3 clients may be required to send USER/PASS, but the proxy never uses
them. Accept any client credentials blindly and always authenticate to the
IMAP backend with the configured BACKEND_IMAP_USER / BACKEND_IMAP_PASS.
Remove the previous fallback that connected to the backend using
client-supplied credentials when backend credentials were unset; the proxy
now raises a clear configuration error in that case.
Add tests covering that client credentials are ignored and that missing
backend credentials are reported.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Emma Thorpe