Extract the decision of whether a push-listen notification must be
delivered at high (urgent) FCM/APNs priority into a static, pure
helper (isHighPriorityPush) so it can be unit-tested without a push
gateway.
Only real-time call signaling — values whose pushType is "audioCall"
or "videoCall", published at high priority (priority == 0) — warrants
an urgent push. Presence/connection-request refreshes (empty
pushType), non-call types and expirations are not urgent.
The helper is not wired in yet; this commit only adds it and its
tests, keeping the current handlePushListen behavior unchanged.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>