Addressing the P1 note from the Codex reviewer: applyCustomHeaders is
also called with a synthetic &http.Request{Header: ...} from the
websockets executors (aistudio_executor.go, codex_websockets_executor.go),
which forward only the header map. The previous continue meant a custom
Host was dropped from that map, regressing virtual-host overrides on
those flows. Mirror the value to both r.Host (for real net/http) and
r.Header (for header-map-only consumers).
Custom headers configured under openai-compatibility (and any other
provider passing through applyCustomHeaders) were silently dropped for
the Host key, because Go's net/http reads the wire Host from
req.Host, not req.Header["Host"]. As a result, virtual-host routed
upstreams (e.g. LiteLLM behind an ingress) saw the base-url's host
instead of the user-configured override and returned 404.
Detect the Host key with http.CanonicalHeaderKey and assign it to
req.Host so it is actually written on the wire. Other headers continue
to use Header.Set as before.
Fixes#2833
muzhi1991