[BUG] `Array::get_backend()` can never return `OPENCL`

[`src/core/array.rs:392-404`](https://github.com/arrayfire/arrayfire-rust/blob/master/src/core/array.rs#L392-L404):

```rust
match (err_val, ret_val) {
(0, 1) => Backend::CPU,
(0, 2) => Backend::CUDA,
(0, 3) => Backend::OPENCL,
_ => Backend::DEFAULT,
}
```

`af_backend` is a bit-flag enum, not sequential (`include/af/defines.h:414-417`, v3.8.0):

```c
AF_BACKEND_DEFAULT = 0,
AF_BACKEND_CPU = 1,
AF_BACKEND_CUDA = 2,
AF_BACKEND_OPENCL = 4
```

There is no backend with value 3. An OpenCL-created array returns 4, falls through to the wildcard, and is reported as `Backend::DEFAULT`.

**Fix:** `(0, 4) => Backend::OPENCL`.

---

Found by Claude Opus 5

從 src/core/array.rs:392-404 開始，並與 include/af/defines.h:414-417 中的後端值進行比較。更新對應，使使用 OpenCL 建立的 array 被回報為 Backend::OPENCL，然後執行現有的測試套件或與後端相關的測試來驗證結果。

由索引模型根據 Issue 內容生成。