Fix Unity importer materials for URP

[sylvesterkaczmarek]

Fixes #3273.

Summary

The Unity asset importer currently creates opaque MJCF materials with Unity's Built-in Standard shader and clones a Built-in transparent material template. In projects using the Universal Render Pipeline, those materials render magenta.

This change:

• detects when URP is the active render pipeline and uses Universal Render Pipeline/Lit;
• configures transparent URP materials with alpha blending;
• maps MJCF color and smoothness properties to the corresponding URP material properties;
• uses a URP-compatible fallback material for geoms without an explicit material;
• preserves the existing Built-in Render Pipeline behavior.

Validation

• Final diff: 1 file, +60/-21 lines, one commit, with no unrelated changes.
• Branch is based on current upstream main at f7da3d295b828478e0171967a7fa3fe6bb2ec922.
• Existing Unity material-import tests exercise the Built-in path. Unity/URP tests were not run locally in this environment, so upstream CI and maintainer validation are required.