Feature Request: Return x-ratelimit-* headers in GitHub Copilot completions API responses

## Summary

GitHub Copilot's OpenAI-compatible completions API does not return `x-ratelimit-*` response headers, making it impossible for API consumers, custom clients, and AI agent frameworks to track token usage, quota consumption, and rate-limit state at runtime.

## Current Behavior

When calling the GitHub Copilot completions API (OpenAI-compatible endpoint), the response contains **no** `x-ratelimit-*` headers:
- `x-ratelimit-limit-requests` — absent
- `x-ratelimit-remaining-requests` — absent
- `x-ratelimit-reset-requests` — absent
- `x-ratelimit-limit-tokens` — absent
- `x-ratelimit-remaining-tokens` — absent

This has been confirmed both via direct API testing and by inspecting session logs from clients that parse these headers. There is also **no REST endpoint** to poll current quota usage (e.g. "how many requests do I have left this hour?") — unlike OpenAI's `/dashboard/billing/usage` or providers like Nous Portal and OpenRouter that return these headers natively.

## Expected Behavior

The completions API should return standard `x-ratelimit-*` headers after every response, consistent with other OpenAI-compatible providers:

```
x-ratelimit-limit-requests: 60
x-ratelimit-remaining-requests: 45
x-ratelimit-reset-requests: 23s
x-ratelimit-limit-tokens: 128000
x-ratelimit-remaining-tokens: 70432
```

## Motivation

1. **AI agent frameworks** parse `x-ratelimit-*` headers to display real-time quota/rate-limit info in status bars and UIs. Without these headers, rate-limit state is always `None` for Copilot sessions, degrading the user experience compared to other providers.

2. **Usage transparency** — developers building on the Copilot API have no programmatic way to know how close they are to their quota limit mid-session. The information simply doesn't exist at runtime.

3. **OpenAI-compatibility** — the API surface implies OpenAI-compatible behaviour. Returning these headers would align Copilot with the de-facto standard that all other OpenAI-compatible providers follow.

## Workaround

None exists for quota transparency at runtime. Switching to Nous Portal, OpenRouter, or OpenAI as provider restores header-based quota tracking immediately.

## Additional Context

- GitHub Copilot enforces quota server-side based on subscription plan.
- This affects any third-party tool, IDE extension, or agent framework that relies on `x-ratelimit-*` headers for adaptive rate limiting or status display (RPM remaining, token budget remaining, reset time).

Issue không nêu tên tệp nào trong repository, test hay entry point; hãy bắt đầu bằng việc xem lại phần kiểm thử API trực tiếp và bằng chứng từ session log được mô tả trong báo cáo. Công việc được xem là hoàn tất khi Completions API trả về các header x-ratelimit-* được yêu cầu sau mỗi response, bao gồm các giới hạn request và token, các giá trị còn lại và thông tin reset.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.