Python generics for State & FwdProps; poetry -> uv

[wdoppenberg]

This started with just adding generics for state or forwarded_props related attributes, but I also changed poetry in favour of uv. I realize this might be overreaching a little bit, so let me know if you want this rolled back.

Inspired by the generics used in pydantic-ai, now e.g. RunAgentInput is as follows:

class RunAgentInput(ConfiguredBaseModel, Generic[StateT, FwdPropsT]):
    """
    Input for running an agent.
    """

    thread_id: str
    run_id: str
    state: StateT
    messages: List[Message]
    tools: List[Tool]
    context: List[Context]
    forwarded_props: FwdPropsT

With generic types:

JSONValue = Union[str, int, float, bool, None, dict[str, Any], list[Any]]
StateT = TypeVar("StateT", default=JSONValue, contravariant=True)
FwdPropsT = TypeVar("FwdPropsT", default=JSONValue, contravariant=True)

This makes it integrate better with type checking in applications / libraries that use these types.

Please raise any worries or thoughts if you have them.