[Router] Derive error status from a failure class; preserve the worker's status (1/3) (#39463)

Co-authored-by: Kangyan Zhou <kangyan.zhou@radixark.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

experimental/sgl-router/monitoring/grafana-dashboard.json

@@ -511,7 +511,7 @@
     {
       "type": "timeseries",
       "title": "Responses by HTTP status",
-      "description": "Client-visible HTTP status codes (2xx/4xx/5xx, 504=stale-cancel).",
+      "description": "Client-visible HTTP status codes. A 504 is any router-side give-up \u2014 the stale-request deadline or an upstream timeout \u2014 or a worker 504 forwarded unchanged; worker_requests_total{outcome} tells them apart.",
       "datasource": {
         "type": "prometheus",
         "uid": "${datasource}"