Make ClientProxy.pauseServices async and drop its completion block

`pauseServices` took a completion block only because it was historically
called from `deinit`, which can't await (the SE-0371 note). `deinit` now
stops the sync itself, so the sole remaining caller is the AppCoordinator,
which can await instead.

Make `pauseServices` async and update the AppCoordinator background-task
and lifecycle call sites to await it, calling the OS-facing completions
(endBackgroundTask / setTaskCompleted) afterwards. This also removes the
`MainActor.assumeIsolated` workaround — awaiting a @MainActor method hops
onto the main actor cleanly — and fixes a latent bug where a nil `self` in
the background app refresh expiration handler would skip setTaskCompleted.

ElementX/Sources/Services/Client/ClientProxy.swift

@@ -431,18 +431,14 @@ class ClientProxy: ClientProxyProtocol {
         }
     }
     
-    func pauseServices(completion: (() -> Void)?) {
+    func pauseServices() async {
         MXLog.info("Pausing services")
         
         if restartTask != nil {
             restartTask = nil
         }
         
-        let task = transitionServices(to: .suspended)
-        Task {
-            await task.value
-            completion?()
-        }
+        await transitionServices(to: .suspended).value
     }
     
     func expireSyncSessions() async {