Trivial Backend implementation that returns :unknown for every probe.
Used when model_agent_backend is :none, :stub, or when Isabelle is
unavailable at runtime. Keeps the agent's plumbing exercised in tests
without any external dependency.
Trivial Backend implementation that returns :unknown for every probe.
Used when model_agent_backend is :none, :stub, or when Isabelle is
unavailable at runtime. Keeps the agent's plumbing exercised in tests
without any external dependency.