Ensure InputSystemObject exists and initializes to prevent nulls in play mode tests

	1.	Editor_InitializeInEditor_EnablesProjectWideActions
	2.	Editor_InputEventsOccurringWhileGoingIntoPlayMode_AreDiscarded
	3.	Editor_LeavingPlayMode_DestroysAllActionStates
	4.	Editor_LeavingPlayMode_DiscardsInputActionAssetChanges
	5.	Editor_LeavingPlayMode_RemovesAllInputUsersAndStopsListeningForUnpairedDeviceActivity

Author: josepmariapujol-unity

Packages/com.unity.inputsystem/Editor/InputSystemEditorInitializer.cs

@@ -407,6 +407,14 @@ private static void OnEditorProjectChanged()
         /// </summary>
         internal static void OnPlayModeChange(PlayModeStateChange change)
         {
+            // Tests may call this directly without going through the full editor initialization path.
+            // Ensure we have a system object to store/restore transient editor state.
+            if (s_SystemObject == null)
+            {
+                s_SystemObject = ScriptableObject.CreateInstance<InputSystemObject>();
+                s_SystemObject.hideFlags = HideFlags.HideAndDontSave;
+            }
+
             switch (change)
             {
                 case PlayModeStateChange.ExitingEditMode: