Surface Flex capacity failures as a distinct terminal error (#47967)

## What changed

- Recognize `flex_unavailable` in HTTP 429 responses and streamed `error` and `response.failed` events. End normal turns without retries and report `Flex capacity unavailable.`
- Expose `flexUnavailable` through the core and app-server protocols and generated schemas and SDK types.
- Keep Flex capacity failures eligible for Guardian review and sampler retries, with a distinct telemetry classification.
- Map unexpected HTTP responses to `HttpConnectionFailed`, preserving their status codes.

## Testing

Add regression coverage for terminal Flex failures over HTTP, SSE, and WebSocket, HTTP 504 status preservation after a stream retry, and protocol serialization. Update Guardian retry and sampler recovery tests to cover Flex failures.

GitOrigin-RevId: f2e03a6d2d36a6d9748c35952e40397f8670d121

codex-rs/cli/src/doctor.rs

@@ -2440,6 +2440,7 @@ fn websocket_error_detail(err: &ApiError) -> String {
         | ApiError::CyberPolicy { .. }
         | ApiError::BioPolicy { .. }
         | ApiError::MisalignmentPolicyViolation { .. }
+        | ApiError::FlexUnavailable
         | ApiError::ServerOverloaded { .. } => format!("handshake error: {err}"),
     }
 }