fix: Add type definitions for task update callbacks

[geojaz]

When trying to run the demo app as per the instructions: https://github.com/a2aproject/a2a-samples/blob/main/demo/README.md

I get an error something like this:

Traceback (most recent call last):
  File "/Users/eric.hole/dev/geojaz/a2a-samples/demo/ui/main.py", line 28, in <module>
    from service.server.server import ConversationServer
  File "/Users/eric.hole/dev/geojaz/a2a-samples/demo/ui/service/server/server.py", line 27, in <module>
    from .adk_host_manager import ADKHostManager, get_message_id
  File "/Users/eric.hole/dev/geojaz/a2a-samples/demo/ui/service/server/adk_host_manager.py", line 34, in <module>
    from hosts.multiagent.host_agent import HostAgent
  File "/Users/eric.hole/dev/geojaz/a2a-samples/samples/python/hosts/multiagent/host_agent.py", line 28, in <module>
    from .remote_agent_connection import RemoteAgentConnections, TaskUpdateCallback
ImportError: cannot import name 'TaskUpdateCallback' from 'hosts.multiagent.remote_agent_connection' (/Users/eric.hole/dev/geojaz/a2a-samples/samples/python/hosts/multiagent/remote_agent_connection.py)

This fixes things up for me

[gemini-code-assist]

Summary of Changes

Hello @geojaz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've addressed an ImportError that occurred when running the demo application by implementing explicit type definitions for task update callbacks. This change involves importing relevant type-related classes and defining new type aliases to correctly specify the signature of these callback functions, ensuring the application's stability and type correctness.

Highlights

• New Type Definitions for Task Callbacks: I've introduced new type definitions, 'TaskCallbackArg' and 'TaskUpdateCallback', to accurately describe the expected arguments and return type for functions that handle task updates. This ensures type consistency and helps prevent runtime errors.
• Dependency Imports: To support the new type definitions, I've added necessary imports for 'Callable' from Python's standard library and 'TaskArtifactUpdateEvent' and 'TaskStatusUpdateEvent' from the 'a2a.client' module.

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[gemini-code-assist]

Code Review

This pull request correctly addresses an ImportError by adding the necessary type definitions for TaskUpdateCallback. The changes are straightforward and effective. I have one suggestion regarding the new TaskUpdateCallback type to improve clarity and prevent potential misuse, as callbacks are often expected to have no return value, but this one is typed to return a Task.

[holtskinner]

@pstephengoogle Can you confirm this is WAI?