NPUW: Introduce a fail-safe submodel + tests (#34888)
No integration into the critical path here yet
### Tickets:
- EISW-207002
### AI Assistance:
- *AI assistance used: yes*
- *Fully generated per spec*
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
