Fix glTF specular extension parsing

[issam3105]

Objective

• Fix a regression introduced by Enable glTF validation for KHR_materials_specular #24571 where KHR_materials_specular was added to the enabled gltf crate features. This was missed because the asset i used for testing did not make the specular parsing regression visually obvious.
• After enabling this feature, the gltf crate parses KHR_materials_specular as a known typed extension, so it is no longer returned by material.extensions(), which only exposes unknown extension data.

Solution

Read KHR_materials_specular through the typed material.specular() API exposed by the gltf crate.

Testing

Reviewers can test visually by loading the Khronos SpecularTest

[kfc35]

looks good to me