Add timeout for remote compaction requests (#23451)

## Why

Remote compaction currently sends a unary `POST /responses/compact` and
waits for the full response before replacing history or emitting the
completed `ContextCompaction` item. Unlike normal `/responses` streaming
requests, this unary compact request had no timeout boundary. If the
backend accepts the request and then stalls before returning a body, the
existing request retry policy never sees a transport error, so the
compact turn can remain stuck after the started item with no completion
or actionable error.

That matches the reported hang shape in issues such as #18363, where
logs show `responses/compact` was posted but no corresponding compact
completion followed. A bounded request timeout gives the existing retry
policy a concrete timeout error to retry instead of letting the user sit
indefinitely on automatic context compaction.

## What

- Add a request timeout to legacy `/responses/compact` calls.
- Size that timeout from the provider stream idle timeout with a
conservative multiplier, so the default compact attempt gets 20 minutes
rather than the 5 minute stream idle window.
- Map API transport timeouts to a request timeout error instead of the
child-process timeout message.

## Testing

- Not run (per request; CI will cover).

codex-rs/codex-api/src/api_bridge.rs

@@ -123,7 +123,7 @@ pub fn map_api_error(err: ApiError) -> CodexErr {
                 status: http::StatusCode::INTERNAL_SERVER_ERROR,
                 request_id: None,
             }),
-            TransportError::Timeout => CodexErr::Timeout,
+            TransportError::Timeout => CodexErr::RequestTimeout,
             TransportError::Network(msg) | TransportError::Build(msg) => {
                 CodexErr::Stream(msg, None)
             }