No converters for Terrain Specular and Terrain Diffuse shaders

Steps to reproduce:

1. Download attached project and open in Unity.
2. Open Render Pipeline Converter: Window >> Rendering >> Render Pipeline Converter.
3. Make sure Target Pipeline is set to Universal Render Pipeline.
4. Select the Material Shader Converter.
5. Press Scan button.
6. Make sure TerrainDiffuse and TerrainSpecular shaders are selected.
7. Press Convert Assets button.
8. Observe: 

No upgrader found for shader Nature/Terrain/Diffuse.

No upgrader found for shader Nature/Terrain/Specular.

Actual results: Terrain Diffuse and Terrain Specular shaders are not convertet.

Expected results: These are built-in shaders, which should be converted to URP alternative Terrain shader.

Reproducible with versions: 

6000.6.0a3

Tested on (OS): 

MacBook Pro (16-inch, 2021), CPU: Apple M1 Max, Memory: 64 GB, macOS Tahoe: 26.1