Improve audio handling for ElevenLabs integration with Twilio

Update ElevenLabsSessionHandler to configure mulaw 8kHz audio output for Twilio compatibility and adjust message parsing for conversation initiation and audio events.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: ed5132b9-a34b-4e4b-851a-cdb825c097cb
Replit-Commit-Checkpoint-Type: full_checkpoint
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