BYOK Azure OpenAI: `429` throttling exhausts all retries in ~0.15s with no effective backoff; retry count/wait not configurable

### Describe the bug

Using a BYOK **Azure OpenAI** provider (`COPILOT_PROVIDER_TYPE=azure`), transient throttling (`429 Too Many Requests`) causes Copilot CLI to burn all 5 retries near-instantly and abort, instead of backing off. This is especially impactful for self-hosted / air-gapped Azure OpenAI deployments with low quota ceilings (e.g. small TPM/PTU allocations), where brief, fully recoverable throttle windows are common.

I hit this **quickly and often** with an Azure-hosted **GPT 5.4** deployment on a **270K token/min** quota — even routine sessions trip the limit and then fail hard instead of waiting out the short throttle window.

Observed output:

```text
Request failed due to a transient API error. Retrying....
Request failed due to a transient API error. Retrying....
Request failed due to a transient API error. Retrying....
Request failed due to a transient API error. Retrying....
Request failed due to a transient API error. Retrying....

Failed to get response from the AI model; retried 5 times (total retry wait time: 0.15 seconds).
```

All 5 retries are consumed in ~0.15s total (as low as ~0.02s each), so a recoverable throttle is treated as a hard failure.

### Affected version

**Copilot CLI version:** 1.0.51

### Steps to reproduce the behavior

1. Configure a BYOK Azure OpenAI deployment with a low rate-limit ceiling (e.g. GPT 5.4 at 270K token/min).
2. Run Copilot CLI with that provider/model and issue requests that exceed the ceiling.
3. Observe 5 immediate retries and abort with a sub-second total wait time.

### Expected behavior

- On `429`/`503` from a BYOK provider, apply effective backoff:
- Respect `Retry-After` when present (Azure OpenAI typically returns it) — confirm this path is wired for the `azure`/BYOK provider, not only GitHub-hosted CAPI.
- When no `Retry-After` is present, fall back to a **default exponential backoff with jitter** (capped at a few minutes) rather than ~0.
- Make retry behavior **user-configurable**, e.g. environment variables such as `COPILOT_RETRY_MAX` and `COPILOT_RETRY_WAIT_MS` for headless/CI use.
- Don't reinterpret a long server-side wait as a transport failure (the ~30–60s client-timeout interaction noted in #2661

### Additional context

- #2760 (closed, v1.0.32) added `Retry-After`-based bounded exponential backoff, but on BYOK **Azure OpenAI** with 1.0.51 the retries still exhaust in ~0.15s — suggesting the backoff isn't applied on the BYOK/`azure` path, or the throttle response is reaching the client without a usable `Retry-After`.
- Docs confirm no retry settings exist today

先追蹤 BYOK Azure provider 對 429/503 回應的處理方式，並將其與 #2760 中新增的 Retry-After backoff 進行比較。使用低配額的 Azure deployment 重現快速重試，然後驗證伺服器提供的 backoff 或 fallback backoff 是否有效，以及提議的 retry 設定是否適用於 headless 使用且不會過早失敗。

由索引模型根據 Issue 內容生成。