refactor(notifications): share markConversationRead helper across receivers

Extract the 'clear unread count + cancel message notification' pair into
a single suspend helper on MeshServiceNotifications so ReplyReceiver,
MarkAsReadReceiver, and ReactionReceiver use one consistent code path.
ReactionReceiver now also clears unread and cancels the notification
once the reaction dispatch succeeds, matching the other receivers.
Receivers that only depended on PacketRepository for this pair drop
that injection.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

core/testing/src/commonMain/kotlin/org/meshtastic/core/testing/FakeMeshServiceNotifications.kt

@@ -67,6 +67,8 @@ class FakeMeshServiceNotifications : MeshServiceNotifications {
 
     override fun cancelMessageNotification(contactKey: String) {}
 
+    override suspend fun markConversationRead(contactKey: String) {}
+
     override fun cancelLowBatteryNotification(node: Node) {}
 
     override fun clearClientNotification(notification: ClientNotification) {}