feat: change Event class generator for EventDelta

[pietroMonta42]

with the new API, now the ThreadMessageDelta is:

class ThreadMessageDelta(BaseModel):
    data: MessageDeltaEvent
    """Represents a message delta i.e.

    any changed fields on a message during streaming.
    """

    event: Literal["thread.message.delta"]
    
 class MessageDeltaEvent(BaseModel):
    id: str
    """The identifier of the message, which can be referenced in API endpoints."""

    delta: MessageDelta
    """The delta containing the fields that have changed on the Message."""

    object: Literal["thread.message.delta"]
    """The object type, which is always `thread.message.delta`."""

[mharrisb1]

Thanks @pietroMonta42 I'll push a fix today or tomorrow whenever I get a second to make the change