Fix event streaming

[creatorrr]

trafficstars

Summary

• refactor event_publisher to flatten logic
• send closing signal when workflows end
• add tests for event streaming

Testing

• ruff format agents-api/agents_api/routers/tasks/stream_transitions_events.py agents-api/tests/test_event_streaming.py
• ruff check agents-api/agents_api/routers/tasks/stream_transitions_events.py agents-api/tests/test_event_streaming.py
• poe typecheck (fails: poe: command not found)
• pytype --config pytype.toml (fails: pytype: command not found)
• poe test (fails: poe: command not found)