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-host/tests/grpc/admission_tests.rs

@@ -2,6 +2,7 @@
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering;
 
+use axum::http::StatusCode;
 use codex_code_mode::GrpcSessionOpenError;
 use pretty_assertions::assert_eq;
 
@@ -9,7 +10,22 @@ use super::*;
 
 #[tokio::test]
 async fn session_admission_retries_only_transient_failures_and_is_bounded() -> Result<()> {
-    for (code, failures, expected_attempts, succeeds) in [
+    let proxy_response = axum::http::Response::builder()
+        .status(StatusCode::SERVICE_UNAVAILABLE)
+        .header(
+            axum::http::header::CONTENT_TYPE,
+            "text/plain; charset=utf-8",
+        )
+        .body("no healthy upstream")?;
+    let mut explicit_status_response = proxy_response.clone();
+    explicit_status_response
+        .headers_mut()
+        .insert(tonic::Status::GRPC_STATUS, "13".parse()?);
+    let html_proxy_response = axum::http::Response::builder()
+        .status(StatusCode::SERVICE_UNAVAILABLE)
+        .header(axum::http::header::CONTENT_TYPE, "text/html")
+        .body("<html>no healthy upstream</html>")?;
+    let cases = [
         (Code::Unavailable, 1, 2, true),
         (Code::ResourceExhausted, 1, 2, true),
         (Code::Unavailable, usize::MAX, 5, false),
@@ -17,7 +33,29 @@ async fn session_admission_retries_only_transient_failures_and_is_bounded() -> R
         (Code::DeadlineExceeded, 1, 1, false),
         (Code::InvalidArgument, 1, 1, false),
         (Code::Unknown, 1, 1, false),
-    ] {
+    ]
+    .into_iter()
+    .map(|(code, failures, expected_attempts, succeeds)| {
+        (
+            tonic::Status::new(code, "transient admission test").into_http::<&'static str>(),
+            code,
+            failures,
+            expected_attempts,
+            succeeds,
+        )
+    })
+    .chain([
+        (proxy_response, Code::Unavailable, usize::MAX, 5, false),
+        (html_proxy_response, Code::Unavailable, 1, 2, true),
+        (
+            explicit_status_response,
+            Code::Internal,
+            usize::MAX,
+            1,
+            false,
+        ),
+    ]);
+    for (response, code, failures, expected_attempts, succeeds) in cases {
         let attempts = Arc::new(AtomicUsize::new(0));
         let count = Arc::clone(&attempts);
         let app = axum::Router::new()
@@ -27,12 +65,12 @@ async fn session_admission_retries_only_transient_failures_and_is_bounded() -> R
             .layer(axum::middleware::from_fn(
                 move |request: axum::extract::Request, next: axum::middleware::Next| {
                     let count = Arc::clone(&count);
+                    let response = response.clone();
                     async move {
                         if request.uri().path().ends_with("/OpenSession")
                             && count.fetch_add(1, Ordering::SeqCst) < failures
                         {
-                            return tonic::Status::new(code, "transient admission test")
-                                .into_http::<axum::body::Body>();
+                            return response.map(axum::body::Body::from);
                         }
                         next.run(request).await
                     }