Scenario JSON rejects `"tools": ""` while the CLI accepts `--tools ""`

[queso]

Found 2026-09-10 running a text-mode compare.

A scenario with "mode": "text", "tools": "" at the top level fails config load with string value must be a non-empty string. The CLI flag --tools "" is documented and accepted. Text mode already implies no tools (defaultTools), so the workaround is to omit the key, but the JSON field should accept the same values the flag does, or the error should name the field and say why.

Repro: any scenario file with "tools": "", then promptdiff compare --scenario it.json.