Update codec config, MIME mapping, and interruption handling

[dhruvladia-sarvam]

• Always send output_audio_codec in WebSocket config (was only in
  model-specific branches, silently omitted for unmatched models)
• Add _codec_to_mime_type() mapping so linear16/mulaw/alaw correctly
  resolve to audio/pcm instead of invalid MIME types
• Expand ALLOWED_OUTPUT_AUDIO_CODECS to include linear16, mulaw, alaw
• Guard send_task and recv_task against ConnectionResetError during
  barge-in so transport-closing errors are suppressed, not re-raised
• Add input_sent_event coordination between send/recv tasks to prevent
  recv from starting before config is sent
• Simplify aclose() to delegate to base class, avoiding race with
  ConnectionPool cleanup