Decides whether a thrown error from _performRequestInternal should
advance the proxy fallback chain. We treat timeouts, network errors, and
5xx HTTP responses as transient — worth retrying against another env. 4xx
errors are legitimate client-side failures (404 not found, 422 validation,
etc.) and would just produce the same response from any other env, so we
surface them immediately.
Decides whether a thrown error from
_performRequestInternalshould advance the proxy fallback chain. We treat timeouts, network errors, and 5xx HTTP responses as transient — worth retrying against another env. 4xx errors are legitimate client-side failures (404 not found, 422 validation, etc.) and would just produce the same response from any other env, so we surface them immediately.