feat: Support list-of-dicts format for multimodal message content

[ppraneth]

This PR resolves a TODO in slime/utils/data.py to support datasets where message["content"] is a list of dictionaries (e.g., [{"type": "image", ...}, {"type": "text", ...}]), which is standard for many multimodal instruction tuning datasets.

Changes

• Updated _build_messages in slime/utils/data.py to handle list inputs.
• Added validation to ensure list items are dictionaries with a valid type.
• Maintained backward compatibility for the legacy string format (text with <image> placeholders).

[ppraneth]

cc @zhuzilin

[ppraneth]

@yitianlian Can you check this pr?