fix(aime): ensure non-empty prompt for API backends

[jayvenn21]

Fix AIME'24 empty prompt for API backends

This PR fixes an issue where AIME'24 tasks generate an empty prompt when evaluated using OpenAI-style / local-completions backends, causing API errors like:

Prompt cannot be empty

HF local models tolerate empty prompts, but API backends do not.

Changes

• Ensure doc_to_text always returns a non-empty prompt (instructional prefix: "Solve the following AIME problem.")
• Applied to aime24, aime, and aime25 with correct dataset fields (Problem / Question / problem)
• Add regression test tests/test_aime_prompt.py to prevent future empty prompts

Related issue

Fixes #3497

[jayvenn21]

Note: this does not force apply_chat_template; API users should still pass it when evaluating chat/instruct models. This PR only guarantees a non-empty base prompt.