judge detectors conversation refactor

[jmartin-tech]

Fix #1344

Corrects role based last_message() selection and correct ETL format for judge detectors based on #1254

Verification

List the steps needed to make sure this thing works

• Supporting configuration file judge_det.yaml

plugins:
  detectors:
    judge:
      Refusal:
        detector_model_type: nim
        detector_model_name: meta/llama-3.1-70b-instruct
        confidence_cutoff: 7

• NIM_API_KEY=<my-api-key> python3 -m garak -m test.Blank -p test.Blank -d judge.Refusal --config judge_det.yaml
• Verify the run succeeds
• Verify all automation tests pass

[leondz]

• note re: land order
• test message request

[leondz]

good catches. both changed areas also altered in #1337, mind land order & that role update propagates. test /should/ catch this in its current form even though it doesn't target these two fixes directly