`Template`s should be able to return `Tool` call results as final answers

[eb8680]

Sometimes we may want Templates to return the result of a Tool call directly as a final answer, rather than always passing through a final LLM API call. This includes #526 as a special case, as well as settings where we want Templates to call out to other generative models (e.g. image or video generation) whose outputs may not be compatible with a provider.

[datvo06]

I can think of three ways to implement this:

1. Is to convert the Image /Video from the Tool back to a message in our message sequence and manipulate the call after to judge whether this is the final answer.
2. Is to add an exception to escape the completion loop, along with an annotation for the tool:

class DirectReturn(BaseException, Generic[T]):
    """Raised by a Tool to return its result directly as the Template's final answer."""
    value: T

@Tool.define
def generate_image(prompt: str) -> Annotated[Image.Image, IsFinalAnswer]:
    """Generate an image from a prompt."""
    return some_pil_image

3. Generate a higher-order function that calls tools and uses that function's output as the final answer.