A2A message:stream against a deployed Agent Engine (reasoningEngines) target truncates after TASK_STATE_WORKING, or fails with 'append=True for nonexistent artifact_id'

## Describe the bug

Calling a genuinely **deployed** Vertex AI Agent Engine (`reasoningEngines`) resource via the A2A protocol's streaming endpoint either:

(a) silently stops after reporting `TASK_STATE_WORKING`, never delivering an artifact or completion event, or
(b) — after upgrading to `a2a-sdk==1.1.2` / `google-adk==2.6.3` on both client and server — fails with an explicit client-side error:

```
A2A request failed: append=True for nonexistent artifact_id='' in task ''.
The artifact must be created (append=False) before appending parts to it.
```

The **non-streaming** `message:send` call against the **identical deployed target**, same agent code, same question, completes successfully and returns the full answer every time. The bug is specific to streaming (`message:stream`) against a deployed Agent Engine target — the same code against a **local** A2A server (plain uvicorn, no Agent Engine) streams correctly with no truncation.

## To Reproduce

1. Deploy an ADK agent to Vertex AI Agent Engine via `client.agent_engines.create(agent_engine=AdkApp(...))` (standard `AdkApp` wrapping, not a BYOD custom container).
2. From a separate client process, call the deployed engine's A2A REST endpoint directly:
```python
from a2a.client.transports.rest import RestTransport
from a2a.types import Message, Part, Role, SendMessageRequest

# base_url = https://{region}-aiplatform.googleapis.com/v1beta1/{resource_name}/a2a
transport = RestTransport(httpx_client=http, agent_card=None, url=base_url)
message = Message(role=Role.ROLE_USER, message_id=..., context_id=..., parts=[Part(text="")])
async for event in transport.send_message_streaming(SendMessageRequest(message=message)):
print(event.WhichOneof("payload"))
```
3. Observed sequence: `task(state=SUBMITTED)` -> `status_update(state=SUBMITTED)` -> `status_update(state=WORKING)` -> **stream ends**. No further events, no exception raised, no `TASK_STATE_COMPLETED`, no artifact.
4. Server-side logging (Cloud Logging on the Agent Engine resource) confirms the agent completes its work in the same window (tool calls, model calls, etc. all succeed server-side) -- the completion event is simply never delivered to the streaming client.
5. Calling the same deployed resource with `message:send` (non-streaming) instead, with an identical request, returns the complete, correct `Task` result immediately, every time.

### Second failure mode (reproduced only after the 1.1.2/2.6.3 upgrade)

Using `google.adk.agents.remote_a2a_agent.RemoteA2aAgent` with `ClientConfig(streaming=True)`, pointed at a second deployed Agent Engine agent that the first agent delegates to (orchestrator -> specialist, both independently deployed as their own `reasoningEngines` resources), on `a2a-sdk==1.1.2` / `google-adk==2.6.3` on both ends:

```
ERROR: A2A request failed: append=True for nonexistent artifact_id='...' in task '...'.
The artifact must be created (append=False) before appending parts to it.
```

Reproduced twice, on two separate calls with different task/artifact IDs, same failure shape both times. Using `ClientConfig(streaming=False)` for the identical delegation call works correctly every time (this is the workaround currently in place).

## Versions tested (all four combinations)

| Client | Server (deployed) | Result |
|---|---|---|
| a2a-sdk 1.1.1 / google-adk 2.5.0 | a2a-sdk 1.1.1 / google-adk 2.5.0 | Truncates after `WORKING` |
| a2a-sdk 1.1.2 / google-adk 2.6.3 | a2a-sdk 1.1.1 / google-adk 2.5.0 (unchanged) | Identical truncation |
| a2a-sdk 1.1.2 / google-adk 2.6.3 | a2a-sdk 1.1.2 / google-adk 2.6.3 | `RestTransport` raw call: identical truncation. `RemoteA2aAgent` delegation call: the "append=True for nonexistent artifact_id" error above |

## Expected behavior

`message:stream` against a deployed Agent Engine target should deliver the same events a `message:send` call resolves to, in the correct order, without a gap between `WORKING` and the final artifact/completion -- matching the behavior already observed against a local (non-Agent-Engine) A2A server with byte-identical agent code.

## Environment

- Deployment: `google-cloud-aiplatform[agent_engines,adk]`, Vertex AI Agent Engine / Reasoning Engine, standard `AdkApp` wrapping (not BYOD custom container)
- Python 3.12
- a2a-sdk: 1.1.1 and 1.1.2 (both tested, both affected)
- google-adk: 2.5.0 and 2.6.3 (both tested, both affected)

## Additional context

This looks like it lives in Agent Engine's own REST-wrapped proxy layer between the client and the deployed container's A2A server, rather than in either Python library directly -- the identical agent code, unmodified, streams correctly end-to-end against a local (non-Agent-Engine) A2A server. Happy to share a minimal repro repo if useful.