[QA] Rendering | KHR_materials_unlit is ignored in Unity Explorer Preview

### 🛠️ **Build version:**
Current Creator Hub / Unity Explorer build

### 📔 **Issue Description:**
GLB files using the standard `KHR_materials_unlit` extension render correctly as Unlit in Creator Hub, but appear as regular Lit/PBR materials in Unity Explorer Preview.

### 👣 **STR:**

1. Import a GLB containing `KHR_materials_unlit`.
2. Place the model in Creator Hub and confirm that it renders as Unlit in the editor viewport.
3. Launch Preview in Unity Explorer and observe the same model.

### ✔️ **Expected Result:**
The material should remain Unlit in Unity Explorer Preview.

### ❌ **Actual Result with evidence:**
The material is rendered as a regular Lit/PBR material in Unity Explorer Preview.

The same GLB renders correctly as Unlit in Creator Hub and in standard glTF viewers.

### ➗ **Reproduction:**

##### 🔁 Always - Happens 100% with the tested GLB.

#### 🖥️ **Operative system and additional Notes:**
Windows 10

The GLB contains a valid `KHR_materials_unlit` material extension.

Example:

```json
"extensions": {
"KHR_materials_unlit": {}
}
```

For testing, other material extensions were also removed so that only `KHR_materials_unlit` remained. The issue still reproduced.

[KHR_materials_unlit_repro.zip](https://github.com/user-attachments/files/31793332/KHR_materials_unlit_repro.zip)

Start in Unity Explorer Preview by importing the attached KHR_materials_unlit_repro.zip and comparing its material handling with Creator Hub. Verify the GLB's KHR_materials_unlit extension is preserved through Preview and confirm done when the model renders as Unlit rather than Lit/PBR.

Written by the indexing model from the issue text.