feat(openai): support codex models via the Responses API (#87)

Azure-hosted GPT-5.x codex models (e.g. gpt-5.3-codex) reject the Chat
Completions API and require the Responses API, so they could not be used
via the OpenAI-compatible provider, which routed everything through
chat.completions.create.

createMessage/completePrompt now detect codex models and route them through
the Responses API (responses.create) with streaming, Anthropic->Responses
input conversion, and flat tool-schema conversion. Non-codex models keep
using chat.completions unchanged. Ports RooCode PR #11952 (unmerged upstream).

Adds openai-codex-responses.spec.ts (13 tests). Existing openai tests
unchanged.

Author: proyectoauraorg