fix timeout and audio sync

[raghavm243512]

Series of small/medium changes to fix timeout and keep audio in sync:

ElevenLabs' Conversational AI changed its end-of-utterance audio delivery: it
no longer stops the user-agent stream in a way the user simulator's partial-chunk
heuristic could detect, so the user sim never emitted end-of-turn / trailing
silence and S2S assistants (ElevenLabs, OpenAI realtime) deadlocked waiting on
each other → inactivity timeout.

• user sim: derive end-of-turn from ElevenLabs' agent_response event (arm) +
  audio-buffer drain (fire), independent of stream shape; emit trailing silence
  so the assistant VAD can close the turn
• elevenlabs bridge: forward µ-law 8kHz in 250ms chunks (was batching 1s chunks)
• elevenlabs server: wall-clock-anchored recording (both tracks, assistant at
  playback time) → correct mixed audio; model_response latency keyed off the
  user_transcript event
• bump elevenlabs SDK to 2.53.0