Guard for the pillar-2 entry points (runHarnessTest/runEval): a pillar-1-only
adapter (Cursor, Devin, Amp, Amazon Q) has no mockable transport, so driving
the deterministic/eval tiers against it would hang or spawn nothing. Calling
this up front turns that into a clear, immediate error. Returns the narrowed
runtime+modelMock so the caller can use them without re-checking for undefined.
Guard for the pillar-2 entry points (runHarnessTest/runEval): a pillar-1-only adapter (Cursor, Devin, Amp, Amazon Q) has no mockable transport, so driving the deterministic/eval tiers against it would hang or spawn nothing. Calling this up front turns that into a clear, immediate error. Returns the narrowed runtime+modelMock so the caller can use them without re-checking for undefined.