fix(voice): restrict stt pipeline reuse to default stt_node

[longcw]

Summary

On agent handoff, _detach_reusable_resources reuses the STT pipeline when type(self.agent).stt_node is type(new_activity.agent).stt_node. That check passes whenever two agents share the same stt_node method object — including via a common base class — but the detached pipeline is a running generator bound to the old agent's self. The default Agent.stt_node is safe because it only reads self/activity at startup, before the yield loop. Any custom override that touches self.session or self.activity inside the loop will crash after handoff:

File "/code/ai_receptionist/logic/livekit_agents/base.py", line 1079, in stt_node
    self.session,
File ".../voice/agent.py", line 684, in session
    return self._get_activity_or_raise().session
File ".../voice/agent.py", line 382, in _get_activity_or_raise
    raise RuntimeError("no activity context found, the agent is not running")

Tighten the precondition to "both agents use Agent.stt_node directly." Custom overrides — even ones shared via inheritance — no longer trigger reuse, since we can't verify they're safe to keep iterating after their bound self loses its activity.

[devin-ai-integration]

✅ Devin Review: No Issues Found

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