* 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).
* test(core): cover malformed material refs
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Co-authored-by: Aymeric Rabot <aymeric.rabot@gmail.com>
