MaterialX shaders break when changing a terminal connection in Hydra 2

**Describe the bug**
In Houdini 21, switching the MaterialX output during IPR breaks the shader.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a simple MaterialX shader with a standard surface

3. Now switch the terminal shader to another standard surface - the shader breaks and the render has to be restarted

**Expected behavior**
The new terminal shader is assigned correctly

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Used Software Versions**
- Houdini 21
- Arnold: [e.g. 7.4.3.0]

**Additional context**
Add any other context about the problem here.

The issue names no source files, tests, or entry points. First reproduce the MaterialX terminal-switch workflow in Houdini 21 during IPR, recording the exact Arnold version and render behavior. Done means switching to another standard surface assigns the new terminal shader correctly without requiring a render restart.

Written by the indexing model from the issue text.