Nanite Skeletal Mesh ignores Hidden Material Section flag

Unlike non-Nanite Skeletal Meshes, Nanite-enabled Skeletal Meshes do not correctly respond to the Show flag passed via ShowMaterialSection(). Consequently, the Isolate checkbox for material slots in the Skeletal Mesh Editor also fails to function properly.

Note: This issue persists even after applying the fix from //UE5/Main CL 45797729, which addressed a related issue.

1. Import a Skeletal Mesh with multiple material slots
2. Enable Nanite on the Skeletal Mesh
3. Open the Skeletal Mesh in the Skeletal Mesh Editor
4. Toggle the "Isolate" checkbox for any material slot → No visual change occurs
5. Place the Skeletal Mesh in a level
6. In the Level Blueprint, call Show Material Section on the Skeletal Mesh Component with Show = false to hide a specific material section
7. Run PIE (Play In Editor) → The hidden section remains visible

Expected Behavior:

• Material sections with Show = false should not be rendered (same behavior as non-Nanite Skeletal Meshes)
• The Isolate checkbox in the Skeletal Mesh Editor should hide/show the corresponding material sections
• Runtime calls to ShowMaterialSection(MaterialID, false) should make the section invisible

Actual Behavior:

• Material sections with Show = false continue to be rendered with their original materials
• No visual change occurs when toggling Isolate or calling ShowMaterialSection(MaterialID, false)