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/response-debug-context/src/lib.rs

@@ -88,6 +88,7 @@ pub fn telemetry_api_error_message(error: &ApiError) -> String {
         ApiError::BioPolicy { .. } => "bio policy".to_string(),
         ApiError::MisalignmentPolicyViolation { .. } => "misalignment policy violation".to_string(),
         ApiError::ServerOverloaded { .. } => "server overloaded".to_string(),
+        ApiError::FlexUnavailable => "flex capacity unavailable".to_string(),
     }
 }