Add plugin to _hotkeyPlugins collection for tracking

Enhanced the plugin management system by adding the `pair` object to the `_hotkeyPlugins` collection in `PluginManager.cs`. This ensures that hotkey-enabled plugins are properly tracked and managed within the `Flow.Launcher.Core.Plugin` namespace.

Author: Jack251970

Flow.Launcher.Core/Plugin/PluginManager.cs

@@ -382,6 +382,7 @@ private static void CheckPluginHotkeys(PluginPair pair)
                     Settings.UpdatePluginHotkeyInfo(GetPluginHotkeyInfo(pair.Metadata.ID));
                 }
                 InitializeWindowPluginHotkey(pair);
+                _hotkeyPlugins.Add(pair);
             }
         }