fixes #9230

Summary

• replace the deprecated assistants helpers with Responses API utilities and new streaming parser tests
• extend provider configuration/environment overrides for response_format, parallel tool calls, and metadata wiring through the API
• normalise chat stream payloads on the Angular side with new specs covering payload coercion

Testing

• npm test (chatapi)
• npm run ng -- test --watch=false --browsers=ChromeHeadless (fails: Angular CLI not available in environment)

https://chatgpt.com/codex/tasks/task_e_68f282cef774832d902d8536bb2e31ea