"Remaining req." should reflect budget usage after pre-allocated Copilot Pro+ credits are exhausted

### Describe the bug

The `Remaining req.` indicator in Copilot CLI correctly tracks usage while consuming pre-allocated Copilot Pro+ credits. However, once those allocated credits are exhausted, the experience breaks down in two ways:

1. **Errors instead of a clear message** — Instead of a friendly notification with actionable guidance, the CLI surfaces raw service errors that are confusing and unhelpful.

2. **No budget visibility** — If the user has additional spending budgets configured (beyond the pre-allocated credits), the `Remaining req.` indicator does not transition to showing budget consumption. It effectively goes dark or becomes meaningless after allocated credits run out.

### Expected behavior

**When pre-allocated credits are exhausted:**

- Seamlessly transition the `Remaining req.` indicator to show budget usage, e.g.: `Pro+ Budget (72% remaining)`
- If no budget is configured, show: `Pre-allocated credits used. Set up a spending budget to continue: [link]`
- Do **not** surface raw backend errors to the user

**When running on a configured budget:**

- The indicator should show percentage remaining: `[budget name] (XX% remaining)`
- The backend presumably already reports this data (similar to how it reports pre-allocated usage), so the CLI just needs to consume and display it

**Budget clarity:**

- When suggesting the user `increase budget,` specify *which* budget (there may be multiple) and link to the relevant settings page

### Additional context

This likely requires deeper integration with the backend Copilot service's billing/quota APIs. The pre-allocated credits flow already works well — the ask is to extend that same level of visibility and grace into the post-allocation / budget-based usage phase.

The transition from `free included credits` to `paid budget` is a critical UX moment. Users need confidence about what they're spending and clear control, not cryptic errors.

Start with the existing Copilot CLI flow that reports pre-allocated credit usage, then trace how quota or billing responses are handled when those credits are exhausted. Reproduce the transition with and without a configured spending budget. Done means the CLI displays the applicable budget and percentage remaining, or an actionable setup message, without exposing raw service errors.

索引モデルが issue の本文から書いたものです。