SessionPrompt restart prohibition

Never call legacy SessionPrompt, including SessionPrompt.prompt(...) or SessionPrompt.loop(...), from V2 admission, execution, continuation, or restart recovery. Admission persists messages and parts separately, so invoking the legacy path can create another assistant message or provider attempt and duplicate tool side effects. Use V2’s durable inbox, serialized runner, and explicit recovery design. Classify abandoned or uncertain provider and tool calls as unknown, expose a model-visible interruption error, and never silently replay or retry them; legacy recovery may only reconcile interrupted records as aborted.