Refactor-Billing-Plan-Engine

The model receives the prompt (and optional system message). The run uses scorer rubric_json_metrics with the JSON configuration below. Pass/fail and partial credit are determined entirely by that scorer against the model output; no human grading.

Return JSON only with keys plan, refactor, tests. Billing plan upgrade, downgrade, proration, and cancellation rules are interleaved in a single nested function. Refactor into cohesive rule units.

{
  "metrics": {
    "intent": {
      "checks": [
        {
          "contains": [
            "upgrade"
          ]
        },
        {
          "contains": [
            "downgrade"
          ]
        },
        {
          "contains": [
            "rule units"
          ]
        }
      ]
    },
    "visible": {
      "checks": [
        {
          "contains": [
            "proration"
          ]
        },
        {
          "contains": [
            "cancellation"
          ]
        },
        {
          "contains": [
            "nested function"
          ]
        }
      ]
    },
    "hidden": {
      "checks": [
        {
          "contains": [
            "billing fixtures"
          ]
        },
        {
          "contains": [
            "edge cases"
          ]
        },
        {
          "contains": [
            "same invoices"
          ]
        }
      ]
    }
  }
}