fix(rustproxy-http): avoid downgrading cached backend protocol on H2 stream errors

rust/crates/rustproxy-http/src/proxy_service.rs

@@ -1388,23 +1388,19 @@ impl HttpProxyService {
                         self.build_streaming_response(upstream_response, route, route_id, source_ip, conn_activity).await
                     }
                     Err(e) => {
-                        // H2 request failed — backend advertises h2 via ALPN but doesn't
-                        // actually speak it. Update cache so future requests use H1.
+                        // H2 request failed on a stream level (e.g. RST_STREAM PROTOCOL_ERROR).
+                        // The H2 handshake succeeded, so the backend genuinely speaks H2 — don't
+                        // poison the protocol cache.  Only handshake-level failures (below) should
+                        // downgrade the cache to H1.
                         let bk = format!("{}:{}", upstream.host, upstream.port);
-                        warn!(
+                        debug!(
                             backend = %bk,
                             domain = %domain,
                             error = %e,
                             error_debug = ?e,
-                            "Auto-detect: H2 request failed, falling back to H1"
+                            "H2 stream error, retrying this request as H1"
                         );
                         self.metrics.backend_h2_failure(&bk);
-                        let cache_key = crate::protocol_cache::ProtocolCacheKey {
-                            host: upstream.host.clone(),
-                            port: upstream.port,
-                            requested_host: requested_host.clone(),
-                        };
-                        self.protocol_cache.insert(cache_key, crate::protocol_cache::DetectedProtocol::H1);
 
                         // Retry as H1 for bodyless requests; return 502 for requests with bodies
                         if let Some((method, headers)) = retry_state {