MaterialX mix_surfaceshader and nodedefs support

**Describe the bug**
Here are 2 simple materialX files that fail to load in Maya 2026. With the first one, it appears that Maya can't load a mix_surfaceshader node. The second test file contains a custom nodedef, and Maya doesn't seem to succeed to load it.
Both graphs are loading and rendering correctly in the MaterialX graph editor.

**Steps to reproduce**
Test files are included in the repro package attached.

Error 1: Load test1.usd using the Create/USD/Stage from file menu in Maya

Error 2: Load test2.usd using the Create/USD/Stage from file menu in Maya

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**Expected behavior**
Screen shots of expected results and the errors are included in the repro package attached.

```
# Warning: Shader not found at for input "fg"
# Warning: Shader not found at for input "bg"
```

```
# Warning: Shader not found at for input "base_color"
```

**Attachments**
Repro package with errors and expected results attached.

**Specs (if applicable):**
- Maya 2026
- mayausd 0.31.0
- MacOS Sequoia 15.4.1

**Additional context**
Does mayaUSD support:
- the mix_surfaceshader node?
- nodedefs?

Thank you so much!!! <3

[matx-issue.zip](https://github.com/user-attachments/files/20019178/matx-issue.zip)