Conference-based transfer for Telnyx with a two-step flow:
- transfer_call dials the destination with a per-transfer webhook URL.
- On call.answered, the webhook seeds a conference with the destination's
live call_control_id and publishes DESTINATION_ANSWERED.
- TelnyxConferenceStrategy joins the caller into the conference on
pipeline teardown (EndTaskReason.TRANSFER_CALL).
- On post-answer destination hangup, the webhook hangs up the caller —
Telnyx's create_conference doesn't accept end_conference_on_exit on
the seed leg, so we tear down the bridge ourselves.
TransferContext gains optional workflow_run_id (for webhook→provider
resolution in multi-config orgs) and conference_id (set on answer,
rd by the strategy).
Also fixes the transfer tool's provider lookup to go through
get_telephony_provider_for_run instead of the deprecated org-default
shim, which was returning the wrong provider in multi-config orgs.
