feat(api): add WebSocket stream IDs (#3612)

## Summary

Adds optional WebSocket-only stream_id routing metadata to Responses
client events and all server response events. Events sharing a stream ID
are processed FIFO and response events echo that identifier.

Source: https://github.com/openai/openai/pull/1282468
OpenAPI commit: 1e5b3b1f1d21b51c5a37d6122c2b3b2d70ef5a1a

## Validation

- Castiron public-baseline generation and formatter completed
- Python source compilation passed
- Strict generated-diff quality review passed
- No private inputs, workflow files, or internal-repository changes are
included