Since #40994 the proxy relays the upstream error body on a 400, so the
public error type is now whatever the upstream sent instead of the
status-derived name. The fake upstream answered every scripted failure
with type api_error, which made the public-error contract in
test_retry_counts_and_public_errors_match_actual_provider_attempts fail
on main. The fake now sends the type a real OpenAI-compatible upstream
sends for the status, so the assertion holds whether the proxy relays
or maps the type
