Fix stale playout state across multi-step voice turns

[jayeshp19]

Summary

This change makes interruption handling depend on whether the current generation is actively in playout, instead of only checking for the presence of a current SpeechHandle.

Changes

• track active playout on the current generation
• clear playout state on _advance_step()
• reuse shared playout start/finish handling in AgentActivity
• add regression tests for started vs unstarted interruption behavior

Verification

• uv run ruff check tests/test_agent_session.py
• uv run pytest tests/test_agent_session.py -k 'advance_step_resets_playout_state or interrupt_by_audio_activity' -q
• uv run pytest tests/test_agent_session.py::test_interruption_before_speaking tests/test_agent_session.py::test_interruption tests/test_agent_session.py::test_tool_call -q

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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