Add support for GitHub Copilot /responses endpoint

[somtooo]

Add support for GithubCopilot /responses endpoint. This gives the copilot chat provider the ability to use the new GPT-5 codex model and any other model that lacks support for /chat/copmletions endpoint.

Closes #38858

Release Notes:

• Add support for GithubCopilot /responses endpoint.

Added

1. copilot_response.rs that has the /response endpoint types
2. uses response endpoint if model does not support /chat/completions.
3. new into_copilot_response() to map LanguageCompletionEvents to Request.
4. new map_stream() to map response stream event to LanguageCompletionEvents and tests.
5. Fixed a bug where trying to parse response for non streaming for /chat/completion was failing

Notes

There is a pr open - #39989 for adding /response support for OpenAi and OpenAi compatible API. Altough they share some similarities (copilot api seems to mirror openAi directly) ive simplified some stuff and tried to keep it the same with the vscode-chat implementation where possible. There might be a case for code reuse but i think keeping them separate for now should be ok.

Tool Calls

Image

[somtooo]

@ConradIrwin tagging for feedback

[ConradIrwin]

Amazing, thank you!

I tested this a bit and streaming/tool calls etc. seem to work. I like that you kept it isolated, so going to merge.