[QA] Bevy editor | Thumbnails not loading for local assets and custom items in asset panel

### 🛠️ **Build version:**
Version 0.47.0 (0.47.0)

### 📔 **Issue Description:**
In the Bevy editor's Scene Editor, thumbnail previews do not render for imported local assets or custom items in the "Local Assets" and "Custom Items" panel tabs. The items themselves load correctly in the viewport and behave as expected when dragged into the scene — only the thumbnail/preview image in the asset panel fails to display, showing a blank/gray placeholder instead.

### 👣 **STR:**

1. Open a scene in Creator Hub with the Bevy editor selected.
2. Import a local asset (e.g. a .glb file) via the "Local Assets" tab, or add an item under "Custom Items".
3. Observe the thumbnail for the imported asset in the asset panel.

### ✔️ **Expected Result:**
The thumbnail/preview image for the imported local asset or custom item should render in the "Local Assets" / "Custom Items" panel tabs.

### ❌ **Actual Result with evidence:**
The item is listed in the panel and functions correctly when dragged into the viewport (confirmed with a local `ZombieMale.glb` asset and a "my platform" custom item), but no thumbnail image loads — a blank/gray placeholder box is shown instead. See attached screenshots (Local Assets tab and Custom Items tab).

### ➗ **Reproduction:**
##### 🔁 Always - reproduced consistently with both a local .glb import and a custom item.

#### 🖥️ **Operative system and additional Notes:**
macOS (M3 Pro) and Windows 11 — Version 0.47.0 (0.47.0)

Split out from #1554 per discussion with Nico; #1554 covers custom items not loading at all in Bevy, while this is specifically about the thumbnail/preview failing to render even though the item loads and functions correctly.