Add per-model `stream: false` option in my-models.yml for non-streaming custom endpoints

Feature Request

Summary

Add a stream configuration option per model in my-models.yml so users can set stream: false for custom endpoints that do not support Server-Sent Events (SSE) streaming.

Current Behavior

AI Toolkit always sends "stream": true when calling custom model endpoints. If the endpoint does not support streaming, the request fails with a 500 error, making the model completely unusable in AI Toolkit even if the endpoint works correctly for non-streaming requests.

Expected Behavior

Allow users to opt out of streaming per model in my-models.yml:

- name: Claude Opus 4.6 v1
  chat_completion_url: https://my-gateway.example.com/v1/chat/completions
  stream: false   # <-- new option
  api_model_name: my-model # e.g. claude-opus-4.6