When a stage filters a non-empty array down to empty, describe the input so the agent can
self-correct a wrong field name / value casing instead of blindly retrying: lists each field
with its distinct values present (capped). E.g. "matched 0 of 150 items. Values present:
status=[Open, Pending, Closed]; priority=[Low, Medium, High, Critical]".
When a stage filters a non-empty array down to empty, describe the input so the agent can self-correct a wrong field name / value casing instead of blindly retrying: lists each field with its distinct values present (capped). E.g. "matched 0 of 150 items. Values present: status=[Open, Pending, Closed]; priority=[Low, Medium, High, Critical]".