Wire background audio mixer to telephony transports

[Copilot]

Connects the existing create_background_sound_mixer utility to telephony transport initialization. Background audio configured in templates was not being applied because the mixer wasn't passed to transport parameters.

Changes

• agent/transport.py: Added audio_out_mixer parameter to get_transport_params(), forwarded to FastAPIWebsocketParams for all telephony providers (Twilio, Exotel, Telnyx, Plivo)

• agent/__init__.py: Called create_background_sound_mixer() in telephony transport setup, passed result to transport params. Daily transport skips mixer creation as DailyParams doesn't support it.

The mixer function already handles template validation, file resolution, and mono channel verification. This change completes the integration by plumbing it through the transport layer.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• astral.sh
  • Triggering command: /usr/bin/curl curl -LsSf REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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