fix(aionrs): handle non-v1 OpenAI-compatible API roots for chat completions

### Summary

After the AionUi / AionCore repo split, the old AionUi PR iOfficeAI/AionUi#2514 no longer applies cleanly, but the runtime bug is still relevant.

Some OpenAI-compatible providers use a versioned API root that is not `/v1`, for example:

- Zhipu: `https://open.bigmodel.cn/api/paas/v4`
- Ark: `https://ark.cn-beijing.volces.com/api/v3`
- Qianfan: `https://qianfan.baidubce.com/v2`

When these providers are used through AionRS, the request can become:

```text
/api/paas/v4/v1/chat/completions
```

Expected:

```text
/api/paas/v4/chat/completions
```

### Current code after the split

AionUi still owns the preset provider list, but AionRS runtime URL resolution now lives in AionCore.

Relevant current AionCore behavior:

- `resolve_aionrs_url_and_compat()` handles:
- `is_full_url`
- Gemini `/v1beta/openai`
- official OpenAI `max_completion_tokens`
- It does not yet set `compat.api_path = "/chat/completions"` for known non-`/v1` OpenAI-compatible roots.
- `crates/aionui-system/src/model_fetcher/url_fixer.rs` already knows these roots for model fetching: `/v2`, `/api/v3`, `/api/paas/v4`.

### Proposed fix

In AionCore, when the mapped provider is `openai` and the base URL path ends with one of the known non-`/v1` API roots:

- `/api/paas/v4`
- `/api/v3`
- `/v2`

set:

```rust
compat.api_path = Some("/chat/completions".to_owned());
```

Keep existing behavior unchanged for:

- official OpenAI `/v1`
- DeepSeek and other normal `/v1` OpenAI-compatible providers
- Gemini
- `is_full_url = true`

### Acceptance criteria

- Zhipu `/api/paas/v4` resolves to root chat completions path.
- Ark `/api/v3` resolves to root chat completions path.
- Qianfan `/v2` resolves to root chat completions path.
- Trailing slash variants are covered.
- Official OpenAI `/v1` does not get this override.
- Existing `is_full_url` behavior from AionCore#307 remains unchanged.

Related:

- Old PR: iOfficeAI/AionUi#2514
- User reports: iOfficeAI/AionUi#2713, iOfficeAI/AionUi#2854