fix(router,handlers): strip trailing slash from rewritten URL + log upstream failures

Two coupled bugs surfaced after 9b0ed0b:

1. url::Url::parse("http://host:port").to_string() normalises the
   empty path to "/", so rewrite_loopback_host was returning
   "http://beast:13131/". Downstream callers then did
   format!("{endpoint}/v1/chat/completions") and produced a
   double-slash path that neuron's axum router 404'd with an empty
   body. Strip the trailing slash in the rewriter so the endpoint is
   a clean base string for concatenation.

2. The anthropic_messages handler returned the upstream's empty body
   to the API caller as `"upstream error: "` with no journal log on
   the cortex side. Operators had no way to see what happened. Add
   warn-level tracing on both upstream failure paths (network error
   and non-2xx) with model, node, target URL, status, and a 512-char
   body snippet. The API response now carries just `"upstream
   returned <status>"` — the implementation detail lives in the log.

Updates the two existing rewrite tests for the no-trailing-slash
output.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/src/handlers.rs

@@ -138,9 +138,10 @@ async fn anthropic_messages(
         }
     } else {
         // Non-streaming: proxy, buffer full response, translate back to Anthropic.
+        let target_url = format!("{}/v1/chat/completions", route.endpoint);
         let upstream_resp = fleet
             .http_client
-            .post(format!("{}/v1/chat/completions", route.endpoint))
+            .post(&target_url)
             .body(openai_body)
             .header("content-type", "application/json")
             .send()
@@ -150,7 +151,14 @@ async fn anthropic_messages(
             Ok(r) => r,
             Err(e) => {
                 metrics::counter!("cortex_request_errors_total", &labels).increment(1);
-                return error_response(502, &format!("upstream request failed: {e}"));
+                tracing::warn!(
+                    model = %model_id,
+                    node = %route.node_name,
+                    target = %target_url,
+                    error = %e,
+                    "anthropic proxy: upstream request failed (network)"
+                );
+                return error_response(502, "upstream request failed");
             }
         };
 
@@ -158,7 +166,16 @@ async fn anthropic_messages(
             metrics::counter!("cortex_request_errors_total", &labels).increment(1);
             let status = upstream_resp.status().as_u16();
             let body = upstream_resp.text().await.unwrap_or_default();
-            return error_response(status, &format!("upstream error: {body}"));
+            let body_snippet = body.chars().take(512).collect::<String>();
+            tracing::warn!(
+                model = %model_id,
+                node = %route.node_name,
+                target = %target_url,
+                status,
+                body = %body_snippet,
+                "anthropic proxy: upstream returned non-2xx"
+            );
+            return error_response(status, &format!("upstream returned {status}"));
         }
 
         let body_bytes = match upstream_resp.bytes().await {