fix(agents): tolerate empty A2A message parts in RemoteA2aAgent

[saurabhhhcodes]

Fixes an IndexError when handling initial A2A TASK_REQUIRED responses that contain no message parts. The handler now checks for empty parts before accessing them and marks all present parts as thought for submitted tasks. Added a unit test covering the empty-parts scenario.\n\nTesting plan:\n- Unit test added: tests/unittests/agents/test_remote_a2a_agent.py (new case in TestRemoteA2aAgentMessageHandling).\n- Run tests locally with dev environment active: PYTHONPATH=src pytest tests/unittests/agents/test_remote_a2a_agent.py -q or run the single new test as needed.\n- CI will run full test matrix; this change is small and focused.

Testing

• Added unit test: tests/unittests/agents/test_remote_a2a_agent.py covering empty-parts TASK_REQUIRED.
• Local run: pytest tests/unittests/agents/test_remote_a2a_agent.py — 70 passed, 81 warnings.
• Change: guard against empty event.content.parts in RemoteA2aAgent._handle_a2a_response to avoid IndexError.

Testing

• Added unit test: tests/unittests/agents/test_remote_a2a_agent.py covering empty-parts TASK_REQUIRED.
• Local run: pytest tests/unittests/agents/test_remote_a2a_agent.py — 70 passed, 81 warnings.
• Change: guard against empty event.content.parts in RemoteA2aAgent._handle_a2a_response to avoid IndexError.

[adk-bot]

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[saurabhhhcodes]

Summary: Fix IndexError in RemoteA2aAgent when A2A TASK responses have empty parts.

What changed:

• Guard against empty event.content.parts when handling TASK_REQUIRED responses.
• Mark any present parts as thought for submitted tasks.

Tests:

• Added unit test in tests/unittests/agents/test_remote_a2a_agent.py covering the empty-parts case.
• Local test run: pytest tests/unittests/agents/test_remote_a2a_agent.py — 70 passed, 81 warnings.

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