Add intelligent interruption handling

[fxhxdxd]

What's added

• Backchannel-aware interruption logic for LiveKit voice agents.
• Configurable vocab for backchannels and interrupt words.
• Example agent demonstrating the behavior.

What was the challenge

• Default VAD pauses on filler words ("yeah/ok/hmm") while the agent is speaking, causing hiccups.
• We need to ignore passive acknowledgements during agent speech but still interrupt on real commands or mixed input.

Why it works

• State-aware filter (BackchannelFilter) plus gating in agent_activity.py: only pause speech when STT confirms the user said something other than a pure backchannel.
• Backchannels while speaking are ignored; commands or mixed phrases interrupt; when silent, all input (including "yeah") is processed.
• Fast path on interim transcripts for interrupt words; otherwise defer to final STT to avoid stutters.
• Vocab is configurable via AgentSession(backchannel_words=..., interrupt_words=...) or env vars LIVEKIT_BACKCHANNEL_WORDS / LIVEKIT_INTERRUPT_WORDS.

How to run (console)

1. Ensure env keys (As .env.local) for your chosen STT/TTS/LLM and LiveKit (LIVEKIT_URL, LIVEKIT_API_KEY, LIVEKIT_API_SECRET).
2. uv run python examples/voice_agents/intelligent_interruption.py console
3. While the agent speaks, say "yeah/ok/uh-huh" → it keeps talking. When silent, say "yeah" → it responds. Say "stop" or "yeah wait" → it interrupts.

How to run (dev with Agents Playground)

1. Set env (As .env.local) : LIVEKIT_URL, LIVEKIT_API_KEY, LIVEKIT_API_SECRET.
2. Start : uv run python examples/voice_agents/intelligent_interruption.py dev.
3. Open https://agents-playground.livekit.io, connect and talk to the agent to verify the backchannel behavior.

Proof

• Video: https://drive.google.com/file/d/10UVOjNTleis-KmO4hsQBLCw8g-pjdtAR/view?usp=drivesdk