[python] Defined SDK events should hardcode the type

[logan-markewich]

It's a little annoying that the defined events expect a single type, but I still have to provide that type when instantiating the event object.

Doesn't work

>>> from ag_ui.core.events import TextMessageStartEvent
>>> TextMessageStartEvent(message_id="hello", role="assistant")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/loganmarkewich/giant_change/llama_index/.venv/lib/python3.12/site-packages/pydantic/main.py", line 253, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for TextMessageStartEvent
type
  Field required [type=missing, input_value={'message_id': 'hello', 'role': 'assistant'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing

Works

>>> TextMessageStartEvent(message_id="hello", role="assistant", type="TEXT_MESSAGE_START")
TextMessageStartEvent(type=<EventType.TEXT_MESSAGE_START: 'TEXT_MESSAGE_START'>, timestamp=None, raw_event=None, message_id='hello', role='assistant')

Should the SDK just hardcode the type since there is only one option? It'd be more user friendly imo

[tylerslaton]

Makes total sense! Should be an easy change should anyone want to implement this. Going to loop in @mme here for his opinion as well.