Add OpenAI gpt-5.2-codex-xhigh model

[jroth1111]

OpenAI's Codex very‑high tier isn't represented in https://models.dev/api.json yet. The OpenAI provider currently lists gpt-5.2, gpt-5.2-chat-latest, and gpt-5.2-pro, but there is no Codex variant (no gpt-5.2-codex, and therefore no way to surface an xhigh preset in clients like opencode).

Request

• Add openai/gpt-5.2-codex (and optionally a convenience alias openai/gpt-5.2-codex-xhigh if you represent presets as separate ids). If you prefer not to add aliases, just adding gpt-5.2-codex is enough; clients can set the xhigh reasoning effort as a per‑model option.

Suggested metadata (mirror gpt-5.1-codex-max, update pricing/knowledge if known)

• id: gpt-5.2-codex (or gpt-5.2-codex-xhigh)
• name: GPT-5.2 Codex (or GPT-5.2 Codex XHigh)
• family: gpt-5-codex
• reasoning: true
• tool_call: true
• attachment: true
• temperature: false
• modalities: { input: ["text", "image"], output: ["text"] }
• limit: { context: 400000, output: 128000 }

This unblocks opencode from surfacing the new Codex tier without custom local overrides.