Twilio voice now uses first-class `Call` records as the source of truth
for call state, instead of storing it on
`conversation.additional_attributes` and `conversation.identifier`. Each
call gets its own record, its own `voice_call` bubble matched by
`call_sid`, and its own conference name keyed off `Call.id`. Multiple
calls on the same conversation (for `lock_to_single_conversation`
inboxes) now work correctly, and the conversation card stays in sync
with the real latest message.
Fixes https://linear.app/chatwoot/issue/PLA-121/lock-to-single-thread
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Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
