fix: Apply filter for inbox when the user is an admin (#11197)

Optimization #11183 missed a condition where the inbox_id filter is
manually passed. Due to the previous change, the inbox filter was being
discarded for admins, although it continued to work correctly for
agents.

This PR includes a fix for that specific case and adds a spec to
explicitly test it.

app/finders/conversation_finder.rb

@@ -86,9 +86,13 @@ class ConversationFinder
     @team = current_account.teams.find(params[:team_id]) if params[:team_id]
   end
 
-  def find_all_conversations
+  def find_conversation_by_inbox
     @conversations = current_account.conversations
-    @conversations = @conversations.where(inbox_id: @inbox_ids) unless @is_admin
+    @conversations = @conversations.where(inbox_id: @inbox_ids) unless params[:inbox_id].blank? && @is_admin
+  end
+
+  def find_all_conversations
+    find_conversation_by_inbox
     filter_by_conversation_type if params[:conversation_type]
     @conversations
   end