Legacy A2A executor finalizes metadata-only responses as status=working instead of completed

[YizukiAme]

Bug Description

The legacy A2aAgentExecutor in a2a_agent_executor.py has several finalization issues:

1. Wrong terminal status: When the final ADK event has no content parts (metadata-only or action-only), finalization produces final=True with status=working instead of completed, leaving clients with a "still processing" Task that never updates.

2. Truncated streaming text: Delta-style text streaming only retains the last chunk; earlier partial text is discarded in the synthesized final artifact.

3. Missing artifact metadata: The synthesized final TaskArtifactUpdateEvent does not carry response metadata, so Task.artifacts[*].metadata is empty.

Expected Behavior

• Finalization without failure/interaction should produce status=completed
• Streamed text chunks should be accumulated for the final artifact
• Final artifact events should propagate response metadata

Affected Files

• src/google/adk/a2a/executor/a2a_agent_executor.py