[WIP] Add support for GitHub Models

[dsyme]

I took a look at getting Claude Code to use Open AI Models via GitHub Models. This would be a very convenient combination as all that is required is a GitHub login with GitHub Models access enabled.

The changes in this PR do the job, this was the testing I did:

GITHUB_TOKEN=`gh auth token` PREFERRED_PROVIDER=github uv run uvicorn server:app --host 0.0.0.0 --port 8082 --reload

and

python tests.py

However this fails because the Azure Open AI filters that underlie GitHub Models triggers a jailbreak error when fed the Claude Code system prompt, which is shame, maybe there's something we can do about this, I'll look into it further

ANTHROPIC_BASE_URL=http://localhost:8082 claude --verbose -p "1+1"