Handle Telnyx Call Control events for the transfer destination leg.

The destination leg is dialed by :meth:TelnyxProvider.transfer_call with this URL as webhook_url. Telnyx sends every event for that leg here. Outcomes:

• call.answered: seed a conference with the destination's live call_control_id, stamp conference_id onto the TransferContext, and publish DESTINATION_ANSWERED so transfer_call_handler can end the pipeline. TelnyxConferenceStrategy then joins the caller into this conference at pipeline teardown.
• call.hangup pre-answer (no conference_id on the context): publish TRANSFER_FAILED so the LLM can recover.
• call.hangup post-answer (conference_id set): the destination left a bridged conference; hang up the caller's leg to tear down the empty bridge (Telnyx's create_conference doesn't accept end_conference_on_exit on the seed leg).

Event references: - call.answered: https://developers.telnyx.com/api-reference/callbacks/call-answered - call.hangup: https://developers.telnyx.com/api-reference/callbacks/call-hangup