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).

跟踪 CLI 的 /models 选择器和自定义提供商配置，找出当前单模型列表的组装位置。首先检查 OpenAI 和 Azure 提供商路径及其现有的错误处理。完成的标准是：成功的 /models 响应提供模型 ID，而 404 或其他错误保留当前的单模型行为；可选元数据和环境变量替代方案仍由设计决定。