fix(voice): preserve turns across activity handoffs

[IanCollection]

What changed

• retain a pipeline user turn accepted while its source activity is being paused/replaced
• transfer the turn to the successor activity and run it through the existing user-turn scheduler
• persist an accepted queued turn exactly once if the successor cannot start or the session closes

Why

During an AgentTask or update_agent() handoff, the source activity blocks scheduling before the successor is fully active. An EOU landing in that window currently follows the scheduling_paused branch, returns True, and clears the recognition transcript without adding it to either activity's history. The user turn disappears.

The queue is owned by the activity and moves across the concrete activity boundary. Successful successors consume it through _user_turn_completed_task; terminal paths use the same history/metrics semantics as session close. Realtime models are intentionally excluded because their input commit lifecycle is remote and is covered separately by #6662.

Impact

Cascade/AgentTask and serialized update_agent() handoffs retain a crossing user turn exactly once instead of silently dropping it.

Design feedback requested

This PR implements the existing TODO in on_end_of_turn by transferring ownership to the successor activity. Feedback is especially welcome on whether activity-owned buffering is the preferred framework boundary for chained handoffs.

Validation

• fail-before real VAD/STT/EOU regression timed out with skipping user input, speech scheduling is paused
• covers AgentTask, consecutive update_agent(), successor close rejection, successor startup failure, and source close
• pytest tests/test_agent_task_handoff_turn.py tests/test_nested_agent_task.py tests/test_update_agent_long_on_enter.py tests/test_agent_task_close_race.py tests/test_foreground_run_tracking.py tests/test_agent_session.py --unit -q — 98 passed
• Ruff check and format check passed
• mypy (livekit.agents) — 206 files passed

Related but distinct: #6662 fixes Realtime paused-speech/overlap behavior; this PR handles local pipeline turn ownership across activity replacement.

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