fix(ios): supply missing push notification sound at handoff (#8443)

Temporarily default missing notification sound until the [gateway
fix](https://github.com/block/buzz/pull/8431) is deployed. Preserve any
sound already provided by APNs.

### Validation

Final build 10151: automation verified the push banner on a real iPhone
12 mini against the unchanged staging gateway, and Tom confirmed hearing
the sound. iOS build, two existing handoff tests, and normal pre-push
passed.

### Before merge

- [x] Verify notification delivery and sound on the final iPhone build.

Replaces [#8429](https://github.com/block/buzz/pull/8429) with the
identical commit to obtain a fresh owner review. The prior review
requested tests; this temporary workaround intentionally adds no tests
or helper at the author's request. [Prior review
response](https://github.com/block/buzz/pull/8429#issuecomment-6256498498).

---------

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/ios/NotificationService/NotificationService.swift

@@ -101,11 +101,13 @@ final class NotificationService: UNNotificationServiceExtension {
   ) {
     resolvedScope = nil
     handoff = BuzzNotificationHandoff(handler: contentHandler)
-    restrictedFallbackContent = Self.restrictedFallback(from: request.content)
     guard let content = request.content.mutableCopy() as? UNMutableNotificationContent else {
       finish(request.content)
       return
     }
+    // Workaround until https://github.com/block/buzz/pull/8431 lands.
+    if content.sound == nil { content.sound = .default }
+    restrictedFallbackContent = Self.restrictedFallback(from: content)
     bestAttemptContent = content
     var cleanUserInfo = content.userInfo
     cleanUserInfo.removeValue(forKey: BuzzPushNavigationTarget.userInfoKey)