[Feat]: Add streaming-aware helper for artifact updates

The helper should be added to a2a/utils/artifact.py. Start by examining the existing new_text_artifact function and the TaskArtifactUpdateEvent class. The new ArtifactStreamer needs to generate a stable UUID, manage state, and produce events with the correct append and last_chunk flags. Update the travel_planner_agent sample to use the new helper as a final step.

Problem

When implementing streaming text output via TaskArtifactUpdateEvent, the current new_text_artifact utility generates a fresh UUID for artifact_id on every call. This makes it impossible to use append=True correctly, since append semantics require a stable artifact_id across chunks.

The sample travel_planner_agent demonstrates this problem — it calls new_text_artifact in a loop:

async for event in self.agent.stream(query):
    message = TaskArtifactUpdateEvent(
        contextId=context.context_id,
        taskId=context.task_id,
        artifact=new_text_artifact(
            name='current_result',
            text=event['content'],
        ),
    )
    await event_queue.enqueue_event(message)

Each iteration produces a new artifact_id, so clients cannot merge chunks into a single artifact. This results in N separate artifact cards in the UI instead of one progressively streamed response.

Describe the solution you'd like

Add a stateful streaming helper to a2a.utils that:

1. Generates a stable artifact_id once on construction
2. Provides an append(text) method that returns a TaskArtifactUpdateEvent with append=True, last_chunk=False
3. Provides a finalize() method that returns a TaskArtifactUpdateEvent with append=True, last_chunk=True

Example API:

from a2a.utils import ArtifactStreamer

streamer = ArtifactStreamer(context_id, task_id, name="response")

async for chunk in llm.stream(prompt):
    await event_queue.enqueue_event(streamer.append(chunk))

await event_queue.enqueue_event(streamer.finalize())

This would live in a2a/utils/artifact.py alongside the existing new_text_artifact and new_artifact helpers.

Describe alternatives you've considered

No response

Additional context

• The travel_planner_agent sample should also be updated to use this helper once available.
• The A2A spec defines append on TaskArtifactUpdateEvent as: "If true, the content of this artifact should be appended to a previously sent artifact with the same ID.". So the current new_text_artifact usage in a streaming loop is a misuse of the spec's intent.

Code of Conduct

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