[Feat]: Send Streaming Message returns `Task` or `Message` as an initial response

[sokoliva]

Is your feature request related to a problem? Please describe.

Send Streaming Message
Stream Response object containing:
Initial response: Task object OR Message object

Currently this is not enforced:
While modifying hello_world client sample to expect Task or Message object as an initial response

async def _handle_stream(
    stream: Any, current_task_id: str | None
) -> str | None:
    async for event in stream:
        print(event)
        if event.HasField('message'):
            print(f'Message: {get_message_text(event.message, delimiter=" ")}')
            continue
        if not current_task_id:
            if not event.HasField('task'):
                raise RuntimeError(
                    'Expected first streaming event to be a Task or Message, '
                )
            current_task_id = event.task.id
            print(f'Task created with id: {current_task_id}')
        if not current_task_id:
            current_task_id = event.task.id

it returned an error:
Expected first streaming event to be a Task or Message.

Describe the solution you'd like

Enforce first streaming event to be Task or Message object by modifying on_message_send_stream method in DefaultRequestHandlerV2. It should check if the current event is the first event and construct a new_task if it is not neither Task nor Message.

Describe alternatives you've considered

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[ishymko]

Fixed by #979.