Shader Generation Regression: Real world units on filenames + target unit not working

## Issue

It seems that units set on filenames does not work (anymore) ? This is a sample from unit tests:
```xml

```
I assume this worked before but I don't think they are part of render tests anymore so unknown when this may have broken.
Units on floats and vectors seem to work.

Unknown what has come of target unit setting but it also appears broken at least from testing with MaterialXlXView -- as it does nothing when setting the target unit and reloading the file. Tried this programmatically from Python API and also does not seem to work.

Start by running the cited image unit test with the filename inputs carrying distance units, then compare shader generation for the foot, meter, mile, and centimeter cases. Also inspect the MaterialXView and Python API paths used to set and reload the target unit. Done means filename units affect generated shaders and target-unit changes are reflected after reload, with regression coverage.

Written by the indexing model from the issue text.