fix(core): guard material-ref parsers against non-string values (Sentry MONOREPO-EDITOR-EM)

getLibraryMaterialIdFromRef and getSceneMaterialIdFromRef only guarded
against null/undefined, then called .startsWith(). When a non-string
material ref reaches them (legacy/malformed wall material slot ref),
.startsWith is undefined -> TypeError: e.startsWith is not a function.

Narrow with typeof !== 'string' -> return null, so a bad ref degrades to
'no library/scene material' instead of throwing during wall material
resolution (packages/viewer wall-materials.ts -> parseMaterialRef).

Author: anton-pascal

packages/core/src/material-library.ts

@@ -4187,13 +4187,14 @@ export function toSceneMaterialRef(id: string) {
 }
 
 export function getLibraryMaterialIdFromRef(materialRef?: string | null) {
-  if (!materialRef) return null
+  if (typeof materialRef !== 'string') return null
   if (!materialRef.startsWith(LIBRARY_MATERIAL_REF_PREFIX)) return null
   return materialRef.slice(LIBRARY_MATERIAL_REF_PREFIX.length)
 }
 
 export function getSceneMaterialIdFromRef(materialRef?: string | null): string | null {
-  if (!materialRef?.startsWith(SCENE_MATERIAL_REF_PREFIX)) return null
+  if (typeof materialRef !== 'string') return null
+  if (!materialRef.startsWith(SCENE_MATERIAL_REF_PREFIX)) return null
   return materialRef.slice(SCENE_MATERIAL_REF_PREFIX.length)
 }