Fix votersCount inflation in multiple-choice polls

increase_vote_count/3 was incrementing votersCount on every vote
activity, causing inflation when a single voter picks multiple options.
Now only increments when the actor is a new unique voter, and preserves
existing votersCount otherwise.

Also adds is_integer guard to voters_count/1 to handle nil safely, and
adds tests for the voters_count clause ordering and edge cases.

lib/pleroma/object.ex

@@ -372,13 +372,21 @@ defmodule Pleroma.Object do
             option
         end)
 
-      voters = [actor | object.data["voters"] || []] |> Enum.uniq()
+      existing_voters = object.data["voters"] || []
+      voters = [actor | existing_voters] |> Enum.uniq()
+      new_voter? = actor not in existing_voters
+      existing_voters_count = object.data["votersCount"]
 
       voters_count =
-        if Map.has_key?(object.data, "votersCount") do
-          object.data["votersCount"] + 1
-        else
-          length(voters)
+        cond do
+          is_integer(existing_voters_count) and new_voter? ->
+            existing_voters_count + 1
+
+          is_integer(existing_voters_count) ->
+            existing_voters_count
+
+          true ->
+            length(voters)
         end
 
       data =

lib/pleroma/web/mastodon_api/views/poll_view.ex

@@ -71,7 +71,7 @@ defmodule Pleroma.Web.MastodonAPI.PollView do
     end)
   end
 
-  defp voters_count(%{data: %{"votersCount" => voters}}), do: voters
+  defp voters_count(%{data: %{"votersCount" => voters}}) when is_integer(voters), do: voters
 
   defp voters_count(%{data: %{"voters" => [_ | _] = voters}}) do
     length(voters)