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/router.rs

@@ -335,7 +335,13 @@ fn rewrite_loopback_host(inference_url: &str, neuron_endpoint: &str) -> Option<S
     let new_host = neuron.host_str()?;
     let mut out = inf.clone();
     out.set_host(Some(new_host)).ok()?;
-    Some(out.to_string())
+    // url::Url::to_string normalises an empty path to "/", which then
+    // breaks downstream callers that do format!("{endpoint}/v1/...")
+    // and produce a double slash. The proxy URL is treated as a base
+    // string that the caller appends paths to, so strip the trailing
+    // slash here.
+    let s = out.to_string();
+    Some(s.trim_end_matches('/').to_string())
 }
 
 #[cfg(test)]
@@ -350,14 +356,14 @@ mod tests {
         );
         assert_eq!(
             out.as_deref(),
-            Some("http://beast.hanzalova.internal:13131/")
+            Some("http://beast.hanzalova.internal:13131")
         );
     }
 
     #[test]
     fn rewrites_loopback_with_distinct_inference_port() {
         let out = rewrite_loopback_host("http://127.0.0.1:8080", "http://beast.lan:13131");
-        assert_eq!(out.as_deref(), Some("http://beast.lan:8080/"));
+        assert_eq!(out.as_deref(), Some("http://beast.lan:8080"));
     }
 
     #[test]