`qt resume` on a run that used a demo model fails

[arschles]

Below is an example qt resume failure:

	qt show 8
	Run 8
		eval: pubmedqa
		status: failed
		created: 2026-06-23T03:21:45.171832Z
		duration: 80.673s
		input: {"model":"demo-builtin","num_samples":1000}
		output: -
		error: invalid builtin input JSON: invalid value: string "demo-builtin", expected expected format "provider:model_id" at line 1 column 43
		
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	Run 'qt show 8 --json' for sample-level details.