Add fail-fast on init for missing provider credentials

internal/server/server.go

@@ -239,17 +239,17 @@ func (s *GatewayServer) handleSyncResponse(w http.ResponseWriter, r *http.Reques
 }
 
 func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.Request, provider providers.Provider, providerMsgs []api.Message, resolvedReq *api.ResponseRequest, origReq *api.ResponseRequest, storeMsgs []api.Message) {
-	w.Header().Set("Content-Type", "text/event-stream")
-	w.Header().Set("Cache-Control", "no-cache")
-	w.Header().Set("Connection", "keep-alive")
-	w.WriteHeader(http.StatusOK)
-
 	flusher, ok := w.(http.Flusher)
 	if !ok {
 		http.Error(w, "streaming not supported", http.StatusInternalServerError)
 		return
 	}
 
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Connection", "keep-alive")
+	w.WriteHeader(http.StatusOK)
+
 	responseID := generateID("resp_")
 	itemID := generateID("msg_")
 	seq := 0