BYOK: populate the /model picker from the provider's /models endpoint

Problem

When the CLI runs against a custom provider (COPILOT_PROVIDER_BASE_URL), exactly one model can be configured (COPILOT_MODEL / COPILOT_PROVIDER_MODEL_ID). In-session, /models shows only that single configured model — there is no way to browse or switch models without quitting and relaunching with --model (which must be guessed, since nothing lists what the provider serves).

Request

For COPILOT_PROVIDER_TYPE=openai (and azure), populate the model picker from the provider's standard OpenAI-compatible catalog endpoint:

GET {COPILOT_PROVIDER_BASE_URL}/models

• Fall back silently to today's single-model behavior when the endpoint 404s or errors.
• data[].id is enough for a useful picker; if present, capabilities.limits / supported_endpoints style metadata could fill the Context column the way the native picker does.

A lighter-weight alternative (or complement): a COPILOT_PROVIDER_MODELS env var accepting a comma-separated id list, for providers whose catalog endpoint can't be reached at startup.

Why it matters

Practically every OpenAI-compatible backend already serves /models — Ollama, vLLM, LiteLLM, Azure, and enterprise LLM gateways. We operate such a gateway: it fronts the Copilot API itself (billing stays on the user's Copilot seat) and serves GET /models with exactly the models the seat's plan/org policy allows — but the CLI never asks, so enrolled users see a one-entry picker and file "the CLI is broken" reports, while the native CLI right next to it shows the full list.

Observed on v1.0.77 and v1.0.78 (linux-x64 and darwin-arm64).

CLI の /models ピッカーとカスタムプロバイダーの設定を追跡し、現在の単一モデルのリストがどこで組み立てられているかを確認します。まず OpenAI および Azure プロバイダーのパスと、既存のエラーハンドリングを確認してください。/models の成功レスポンスでモデル ID が返され、404 やその他のエラーでは現在の単一モデルの動作が維持されれば完了です。オプションのメタデータと環境変数による代替手段は、引き続き設計上の選択とします。