Unity: Retain subscriptions when EnteredEditMode is invoked

[mkc1370]

Summary
Subscriptions created in OnEnable or Start are cleared when Unity re-enters Edit Mode because UnityFrameProvider.Clear() is called during PlayModeStateChange.EnteredEditModeand.
As a result, setting up new subscriptions when Edit Mode is entered again becomes difficult.

Observed order (Unity 6000.1.10f1, 2022.3.9f1)

ExitingPlayMode   ← Clear() [to-be]
OnEnable
Start
EnteredEditMode   ← Clear() [as-is]

Minimal repro

using R3;
using UnityEditor;
using UnityEngine;

[ExecuteAlways]
public class R3Test : MonoBehaviour
{
    [InitializeOnLoadMethod]
    private static void Init()
    {
        EditorApplication.playModeStateChanged += state =>
        {
            Debug.Log($"Play mode state changed: {state}");
        };
    }

    private void Start()
    {
        Debug.Log(nameof(Start));
        Observable.EveryUpdate(UnityFrameProvider.PreLateUpdate, destroyCancellationToken)
                  .Take(5)
                  .Subscribe(_ => Debug.Log($"Subscribe in Start: {Time.frameCount}"));
    }

    private void OnEnable()
    {
        Debug.Log(nameof(OnEnable));
        Observable.EveryUpdate(UnityFrameProvider.PreLateUpdate, destroyCancellationToken)
                  .Take(5)
                  .Subscribe(_ => Debug.Log($"Subscribe in OnEnable: {Time.frameCount}"));
    }

    private void OnDestroy() => Debug.Log(nameof(OnDestroy));
}