Preview purpose bound materials don't get converted to maya shaders on import

Hi,

When importing a usd file which contains materials that are bound with the binding:preview the USD materials don't get converted back in to maya shaders.

Default binding without selected Purpose.

` rel material:binding = `

Binding with purpose preview

` rel material:binding:preview = `

**Steps to reproduce**
1. import USD file which contains binding:preview
2. make sure check materials in USD import options
3. Notice materials are missing / not present in the scene.

Alternative method:
1. create usd stage
2. create xform
3. reference USD file which contains binding:preview
4. Right click on xform and Duplicate as Maya Data
5. Notice materials are missing / not present in the scene.

**Expected behavior**
Materials are converted into maya shaders when binding has purpose preview.

**Attachments**

[usd_preview_assignements.zip](https://github.com/user-attachments/files/20982076/usd_preview_assignements.zip)

**Specs (if applicable):**
- OS & version: e.g. Windows 11
- Maya version: Maya 2025
- Maya USD: 0.30.0

Thanks for the help and having a look.

Best,

- Tom