Feature Request: Support xAI Grok Models via OpenAI Responses Format

[RamAnanth]

Currently xAI Grok Models can be used through Claude Code but not yet through Codex which uses the OpenAI Responses Format

[Shriiii01]

PR #73 implements this: xAI Grok models are now supported via the OpenAI Responses format (/v1/responses), so Codex can use them. The gateway routes by model to the xAI Responses API. Ready for review.