Task was specified but does not exist

### What happened?

a2a_mcp/src/a2a_mcp/common/workflow.py", line 112, in run_node
async for chunk in response_stream:
| ...<6 lines>...
| yield chunk
a2a.client.errors.A2AClientJSONRPCError: JSON-RPC Error code=-32001 data=None message='Task xxx was specified but does not exist'
latest a2a_mcp，occured by orchestrator_agent, task is build by new_task, so issue:373 is helpless for me. Host : cli/a2a_gui

### Relevant log output

```shell
+-+---------------- 1 ----------------
| Traceback (most recent call last):
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/sse_starlette/sse.py", line 240, in cancel_on_finish
| await coro()
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/sse_starlette/sse.py", line 159, in _stream_response
| async for data in self.body_iterator:
| ...<10 lines>...
| raise SendTimeoutError()
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/server/apps/jsonrpc/jsonrpc_app.py", line 436, in event_generator
| async for item in stream:
| yield {'data': item.root.model_dump_json(exclude_none=True)}
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/server/request_handlers/jsonrpc_handler.py", line 142, in on_message_send_stream
| async for event in self.request_handler.on_message_send_stream(
| ...<13 lines>...
| )
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/server/request_handlers/default_request_handler.py", line 349, in on_message_send_stream
| await self._cleanup_producer(producer_task, task_id)
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/utils/telemetry.py", line 194, in async_wrapper
| result = await func(*args, **kwargs)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/server/request_handlers/default_request_handler.py", line 364, in _cleanup_producer
| await producer_task
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/utils/telemetry.py", line 194, in async_wrapper
| result = await func(*args, **kwargs)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/server/request_handlers/default_request_handler.py", line 172, in _run_event_stream
| await self.agent_executor.execute(request, queue)
| File "agents/a2a_mcp/src/a2a_mcp/common/agent_executor.py", line 51, in execute
| async for item in self.agent.stream(query, task.context_id, task.id):
| ...<46 lines>...
| )
| File "agents/a2a_mcp/src/a2a_mcp/agents/main_agent.py", line 138, in stream
| async for chunk in self.graph.run_workflow(
| ...<80 lines>...
| yield chunk
| File "agents/a2a_mcp/src/a2a_mcp/common/workflow.py", line 171, in run_workflow
| async for chunk in node.run_node(query, task_id, context_id):
| ...<18 lines>...
| yield chunk
| File "agents/a2a_mcp/src/a2a_mcp/common/workflow.py", line 112, in run_node
| async for chunk in response_stream:
| ...<6 lines>...
| yield chunk
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/client/legacy.py", line 125, in send_message_streaming
| async for result in self._transport.send_message_streaming(
| ...<6 lines>...
| )
| File "agents/a2a_mcp/.venv/lib/python3.13/site-packages/a2a/client/transports/jsonrpc.py", line 165, in send_message_streaming
| raise A2AClientJSONRPCError(response.root)
| a2a.client.errors.A2AClientJSONRPCError: JSON-RPC Error code=-32001 data=None message='Task cd096d4e-19aa-4459-82fa-aea563b9e722 was specified but does not exist'
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The error originates in a2a_mcp/src/a2a_mcp/common/workflow.py line 112. Examine the task creation and lookup logic in the orchestrator_agent and new_task functions. Check how task IDs are generated and stored versus how they are retrieved. The issue references a CLI/GUI host and mentions issue #373 is not helpful, so review related code changes.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.