feat(a2a): wire artifact delivery interceptor in fast_api A2A setup

[Venseven]

Summary

include_artifacts_in_a2a_event_interceptor was added in commit e63d991 to allow artifact delivery on the A2A path. However it was never wired — fast_api.py constructs A2aAgentExecutor with no config, so execute_interceptors defaults to None and artifact_delta is silently dropped.

This PR passes include_artifacts_in_a2a_event_interceptor as the default interceptor when setting up A2A agents in fast_api.py.

Changes

• fast_api.py: pass A2aAgentExecutorConfig(execute_interceptors=[include_artifacts_in_a2a_event_interceptor]) when constructing A2aAgentExecutor

Behaviour

• Before: agents that call artifact_service.save_artifact() emit no TaskArtifactUpdateEvent on the A2A path — artifact_delta is dropped
• After: saved artifacts are delivered as TaskArtifactUpdateEvent on both message/send and message/stream paths

Non-breaking

The interceptor is a no-op when artifact_delta is empty or artifact_service is None. Users who want to opt out can pass execute_interceptors=[] explicitly.

Testing

Verified end-to-end with a LangGraph-based agent using google-adk==1.31.0: artifacts saved during an agent run now appear as TaskArtifactUpdateEvent in A2A message/send responses after this change.

Fixes #5379
Relates to #3630

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[adk-bot]

Response from ADK Triaging Agent

Hello @Venseven, thank you for your contribution!

Before we can merge this PR, you need to sign the Contributor License Agreement (CLA). You can do so by following the instructions in the comment from google-cla.

Also, could you please provide the console output or logs from your end-to-end testing? This will help the reviewers to verify the fix more efficiently.

Thanks!