Fix code-mode session admission retries for proxy HTTP 503 responses (#61153)

## Why

Proxy HTTP 503 responses can contain plain-text or HTML bodies instead of gRPC frames. These responses should reach the existing session admission retry logic as `Unavailable` errors.

## What changed

Strip body data from HTTP 503 responses to `OpenSession`, preserving headers and trailers so tonic can honor an explicit gRPC status or infer `Unavailable` when none is present. Keep this handling scoped to session admission and retain the existing five-attempt retry limit.

## Testing

Extend admission tests to cover bounded retries for plain-text 503 responses, recovery after an HTML 503 response, and an explicit `Internal` gRPC status that prevents retries.

GitOrigin-RevId: c996e13506464e8b65bae06172324bcbadb1f394

codex-rs/code-mode/src/grpc_session/mod.rs

@@ -59,6 +59,9 @@ mod transport;
 
 type GrpcClient = CodeModeHostClient<GrpcTransport>;
 
+#[derive(Clone)]
+struct SessionAdmission;
+
 const SHUTDOWN_ERROR: &str = "code mode session is shutting down";
 const ROUTE_HEADER: &str = "x-code-mode-route";
 // Bound result-delivery RPCs, not running tools. A full completion burst can
@@ -184,6 +187,7 @@ impl GrpcCodeModeSessionProvider {
                         }
                     },
                 });
+                request.extensions_mut().insert(SessionAdmission);
                 inject_span_traceparent(&mut request, &open_session_span);
                 let result = async {
                     let response = client.as_ref().clone().open_session(request).await?;