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

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}"

experimental/sgl-router/src/proxy/mod.rs

@@ -170,10 +170,15 @@ impl Proxy {
         let bytes = match resp.bytes().await {
             Ok(b) => b,
             Err(e) => {
+                // Walk the full source chain (`{:#}`) like the connect-error
+                // handler in `classify_reqwest_error_for` — a mid-body drop's
+                // real cause (incomplete message, connection reset) lives in the
+                // wrapped source, not the outer reqwest error.
+                let cause = anyhow::Error::new(e);
                 tracing::warn!(
                     upstream = %url,
                     status = %status,
-                    error = ?e,
+                    error = %format_args!("{cause:#}"),
                     "upstream dropped connection mid-body",
                 );
                 breaker.record_failure();