fix(agents): record artifact_delta from before/after agent callbacks

[AkshayShah03]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Artifacts saved in before_agent_callback / after_agent_callback are never recorded in an event (artifact_delta dropped) #7478

Problem:
BaseAgent._handle_agent_callbacks only returns an event when a callback returns content or callback_context.state.has_delta() is true. An artifact saved via callback_context.save_artifact() in a before_agent_callback / after_agent_callback that returns None is stored by the artifact service, but its artifact_delta is never put on any event. The session has no record of it, so Runner.rewind_async cannot revert it and event-based artifact surfacing (web UI, A2A) never sees it.

Solution:
Also emit the event when callback_context._event_actions.artifact_delta is non-empty. This matches the workflow node runner, which already checks both state and artifact deltas, and the model/tool callback paths, which already record artifacts saved there.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added to tests/unittests/agents/test_base_agent.py:

• test_run_async_before_agent_callback_records_artifact_delta
• test_run_async_after_agent_callback_records_artifact_delta

Each saves an artifact from the callback (returning None) and asserts an event carries artifact_delta == {'<name>': 0}. Both fail on main (only the agent's own response event is produced) and pass with this change.

$ pytest tests/unittests/agents tests/unittests/flows tests/unittests/runners
2215 passed, 1 skipped, 5 xfailed

$ pytest tests/unittests -n auto
18347 passed, 4 failed

The 4 failures are in tests/unittests/models/test_litellm.py log-level assertions; they pass when run alone or as their own file, and with this change stashed, so they are unrelated to this PR.

pyink and isort report no changes.

Manual End-to-End (E2E) Tests:

Ran the reproduction script from #7478 through a real Runner with InMemorySessionService and InMemoryArtifactService:

• on main: artifact_delta in session events: []
• with this change: artifact_delta in session events: [{'report.txt': 0}]

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.