fix(a2a): report non-terminal stream truncation

[daleselaji-dev]

Summary

RemoteA2aAgent currently treats a cleanly closed A2A stream as success even when the last task status is still non-terminal. Track the last task and emit an explicit error event when the stream ends in a working or submitted state.

This preserves already-yielded events while making truncated remote invocations observable to consumers. Terminal completed, failed, and canceled states remain unchanged.

Validation

• uv run --extra test pytest tests/unittests/agents/test_remote_a2a_agent.py -q (155 passed)
• Ruff check passed for the changed production file
• git diff --check

Fixes #6585

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