fix(legacy): read JudgeRubric questions from Pydantic messages

[crazywriter1]

Supersedes #2005 — same fix after #2303 moved v0 rubrics to verifiers/legacy/.

Summary

• JudgeRubric.judge() only extracted question from dict prompts (isinstance(..., dict)).
• Rollouts normalize prompts to Pydantic Message objects, so the Question block was silently empty.
• Read content via parser helpers (_message_field / _content_to_text) for dict and Message prompts.

Test plan

• Manual: dict prompt still yields last user content as {question}
• Manual: Pydantic-like message with .content no longer produces empty Question
• Manual: multimodal text parts joined into question string

Note

Fix JudgeRubric.judge to read question content from Pydantic messages

Previously, JudgeRubric.judge in judge_rubric.py extracted the last message's content via raw dict access, which failed for Pydantic message objects. It now uses self.parser._message_field and self.parser._content_to_text to normalize the content, consistent with how the rest of the parser handles messages. Behavioral Change: the extracted question string used to format judge_prompt (and its cache key) may differ when prompts contain Pydantic message objects.

^{Macroscope summarized adb8c94.}