Add VoicemailDetector to warm transfer example

[jamsea]

Summary

• Adds VoicemailDetector to the daily-pstn-warm-transfer bot so it can detect when a dialout reaches voicemail
• When voicemail is detected, the bot leaves an automated callback message instead of trying to brief a voicemail greeting
• Uses a separate classifier LLM instance for voicemail detection

Test plan

• Verify bot starts without import errors
• Test warm transfer to a number that goes to voicemail — bot should detect and leave a message
• Test warm transfer where agent answers — should proceed normally (conversation detected)

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