Controls when a ringing call transitions to RingingState.Active.

Implement this to insert custom logic (e.g. waiting for user confirmation) between the publisher peer connection becoming ready and the call going active. Has no effect on non-ringing joins (livestream, direct join).

Concurrent Call.join callers share one in-flight join. Interceptor ownership is first non-cancelled wins: the first join(interceptor) waiter whose coroutine is still not cancelled supplies the interceptor. If that waiter is cancelled (Activity destroyed / recreated) before the interceptor runs, the next still-active waiter's interceptor is used. join(null) does not erase an earlier interceptor. Once callReadyToJoin starts, the selected interceptor is frozen for that invocation.