run_prompt_nodes fell back to every PromptManager node whenever no positive
node was traced, including graphs that do have a sampler and whose only
PromptManager node feeds its negative input. Fall back only when the graph
has no recognisable sampler (new has_sampler helper). Samplers named like
one are recognised even without a model link. Adversarial graphs (cycles,
10k-node fan-out, non-dict inputs) are covered.