Add upstream model list fetching and a Codex-specific model section

internal/gateway/server.go

@@ -75,6 +75,7 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("POST /v1/messages/count_tokens", s.withAuth(s.handleCountTokens))
 	mux.HandleFunc("POST /v1/responses", s.withAuth(s.handleResponses))
 	mux.HandleFunc("GET /api/status", s.handleStatus)
+	mux.HandleFunc("POST /api/fetch-models", s.handleFetchModels)
 	mux.HandleFunc("GET /api/providers", s.handleListProviders)
 	mux.HandleFunc("POST /api/providers", s.handleCreateProvider)
 	mux.HandleFunc("PUT /api/providers/{id}", s.handleUpdateProvider)