Graph Editor: loading material with invalid connections / type mismatches

Hi,

again codegen produced some invalid MaterialX code where we connect a color4 node to a color3 output.

The GraphView happily displays this connection when opening the file


I'm only getting an error when trying to pipe the output then into the shader


Problematic material:
```mtlx

```

Also attaching a video illustrating the issue
note that the invalid connection is correctly refused in the UI when trying to do it but not when loading the graph
also note that when I connect the subgraph output to the shader we get compiler errors and rendering breaks due to the type mismatch

https://github.com/AcademySoftwareFoundation/MaterialX/assets/5083203/62136046-aa6e-4142-b01c-cfd0ef7380e3

Start by loading the problematic MaterialX document in the GraphView and trace the graph-loading path for connection validation. Compare its behavior with the UI's refusal of the same color4-to-color3 connection; done means invalid connections are rejected during loading before they reach shader compilation or rendering.

Written by the indexing model from the issue text.