Fix 'test push loop back' notification check (#5541)

Include a fake client secret as the HS would do, since it's now mandatory.

libraries/push/impl/src/main/kotlin/io/element/android/libraries/push/impl/pushgateway/PushGatewayDevice.kt

@@ -21,5 +21,14 @@ data class PushGatewayDevice(
      * Required. The pushkey given when the pusher was created.
      */
     @SerialName("pushkey")
-    val pushKey: String
+    val pushKey: String,
+    /** Optional. Additional pusher data. */
+    @SerialName("data")
+    val data: PusherData? = null,
+)
+
+@Serializable
+data class PusherData(
+    @SerialName("default_payload")
+    val defaultPayload: Map<String, String>,
 )

libraries/push/impl/src/main/kotlin/io/element/android/libraries/push/impl/pushgateway/PushGatewayNotification.kt

@@ -20,5 +20,5 @@ data class PushGatewayNotification(
      * Required. This is an array of devices that the notification should be sent to.
      */
     @SerialName("devices")
-    val devices: List<PushGatewayDevice>
+    val devices: List<PushGatewayDevice>,
 )

libraries/push/impl/src/main/kotlin/io/element/android/libraries/push/impl/pushgateway/PushGatewayNotifyRequest.kt

@@ -42,9 +42,12 @@ class DefaultPushGatewayNotifyRequest(
                     devices = listOf(
                         PushGatewayDevice(
                             params.appId,
-                            params.pushKey
+                            params.pushKey,
+                            PusherData(mapOf(
+                                "cs" to "A_FAKE_SECRET",
+                            ))
                         )
-                    )
+                    ),
                 )
             )
         )