fix(a2a): mark failed A2A tasks as error events

[sakiko-toyokawa]

Fixes #6708

Convert failed A2A task responses and streaming status updates into ADK error events instead of regular conversation content. The fix covers both legacy and v2 RemoteA2aAgent handlers, handles Task and TaskStatusUpdateEvent shapes, uses _compat.is_failed_status() for a2a-sdk version compatibility, and sets error_code=A2A_TASK_FAILED plus error_message.

Testing plan:

• pytest tests/unittests/a2a: 447 passed, 45 skipped
• pytest tests/unittests/agents/test_remote_a2a_agent.py: 174 passed
• ruff, pyink --check, git diff --check: passed

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