feat(a2a): save artifacts returned by RemoteA2aAgent to the orchestrator session

[vaibhav-patel]

Summary

Fixes #3145.

When a RemoteA2aAgent receives artifacts (e.g. files) from a remote A2A agent, they are now persisted into the orchestrator (parent) session's artifact service, so downstream/sibling agents can load them via context.load_artifact(...).

Previously, artifacts in an A2A response were only flattened into the event's content parts (convert_a2a_task_to_event) and never written to the artifact service, so the orchestrator had no durable, named handle to the returned files.

What changed

• New best-effort helper RemoteA2aAgent._save_a2a_artifacts_to_session(...) that:
  • saves each artifact's blob-like part (FilePart / FileWithBytes / FileWithUri, or inline data) under the artifact's name (falling back to its artifact_id);
  • records the saved version on the event's actions.artifact_delta;
  • is a no-op when no artifact service is configured, and never breaks A2A response handling on a per-artifact failure.
• Wired into both the legacy (_handle_a2a_response) and v2 (_handle_a2a_response_v2) handlers, covering full-task responses (task.artifacts) and streaming artifact updates (TaskArtifactUpdateEvent.artifact).
• This is the client-side counterpart to the existing server-side include_artifacts_in_a2a_event interceptor, completing the artifact round-trip across the A2A boundary.

Tests

• New TestRemoteA2aAgentArtifactPersistence (7 tests) using a real InMemoryArtifactService and real A2A objects, asserting artifacts land in the service and artifact_delta is populated; covers inline bytes, file-by-URI, multiple artifacts, artifact_id fallback, no-service no-op, and text-only (skipped) artifacts.
• All existing RemoteA2aAgent and a2a unit tests pass (467 passed locally).

Note: experimental A2A surface (@a2a_experimental); behavior is additive and gated on an artifact service being present.

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[vaibhav-patel]

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