Prefered language is ignored

- Copilot Chat Extension Version: any
- VS Code Version: latest
- OS Version: linux
- Feature (e.g. agent/edit/ask mode): inline completions
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): any
- Logs: none not required

Steps to Reproduce:

1. type async main (){ ... } save as .rs file
2. change language to javascript and save file again

well my first question is, whats the differnece between https://proxy.individual.githubcopilot.com/chat/completions
and
https://proxy.individual.githubcopilot.com/v1/engines/gpt-41-copilot/completions

what kind of purpose do both of them serve, considering https://proxy.individual.githubcopilot.com/v1/engines/gpt-41-copilot/completions is called every time i do a keystroke, however https://proxy.individual.githubcopilot.com/chat/completions is barely called and i cant even understand when its called or why but it produces better results

and second one is the bug itself, selected language must be prefered