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).

Segui il picker /models della CLI e la configurazione dei provider personalizzati per trovare dove viene assemblato l'attuale elenco con un singolo modello. Inizia controllando i percorsi dei provider OpenAI e Azure e la relativa gestione degli errori esistente. Il lavoro è completato quando una risposta /models riuscita fornisce gli ID dei modelli, mentre i 404 o altri errori mantengono l'attuale comportamento con un singolo modello; i metadati opzionali e l'alternativa tramite variabile d'ambiente restano scelte di progettazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.