Message Parts API

[zhudotexe]

This PR introduces the MessagePart API, which allows messages to be comprised of a number of MessageParts rather than a singular str. To accomplish this, the following changes were made:

• ChatMessage.content now has the type str | tuple[MessagePart | str, ...] | None
  • For base engines (e.g. OpenAI, LLaMA), this will not affect their functionality and they will continue to operate as in kani v0.4
  • content has to be a tuple to enforce immutability; constructors and copy methods enforce this/cast Sequences to the right underlying type
• Added ChatMessage.text: str and ChatMessage.parts: list[MessagePart | str] to get a consistent type from the message content
  • These are properties that proxy the underlying content; .part returns a list proxy of the underlying tuple content or returns a one-element list of the string content
• Refactored a bunch of places across the library where .content was assumed to be a str to use .text instead
• Added a Chain of Thought example using MessageParts to demonstrate how to use the interface

This will lay the foundation for future implementation of multimodal models and engines with engine-specific metadata (e.g. ReAct).

[zhudotexe]

We discussed these comments IRL and will make an update to _auto_truncate_message in a future update that will also allow function calls to return ChatMessages rather than just strings.