Refactor-Guard-Clause-Login

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 refactor, rationale, tests. Refactor a nested login validator with three early rejection cases and one success branch. Preserve exact error messages and return shape.

{
  "metrics": {
    "visible": {
      "checks": [
        {
          "contains": [
            "guard clause"
          ]
        },
        {
          "contains": [
            "early return"
          ]
        },
        {
          "contains": [
            "error messages"
          ]
        }
      ]
    },
    "hidden": {
      "checks": [
        {
          "contains": [
            "return shape"
          ]
        },
        {
          "contains": [
            "success branch"
          ]
        },
        {
          "contains": [
            "no behavior change"
          ]
        }
      ]
    },
    "intent": {
      "checks": [
        {
          "contains": [
            "reduce nesting"
          ]
        },
        {
          "contains": [
            "readability"
          ]
        },
        {
          "contains": [
            "control flow"
          ]
        }
      ]
    }
  }
}