fix: improve image rubric grading

[jameshiester-oai]

Summary

• Ground image-related llm-rubric grading in actual image payloads and visual content instead of accompanying text or attachment presence alone.
• Strengthen multimodal grader instructions so attached images are inspected directly before deciding compliance, refusal, or policy safety.
• Add local and remote matcher coverage for image-related outputs with and without image payloads.

Test plan

• npm test -- test/matchers/llm-rubric.test.ts