MM-36894: Racy test: TestExecuteCommandInDirectMessageChannel (#17881)

* MM-36894: Racy test: TestExecuteCommandInDirectMessageChannel

We were using the pointer to the same post object
in multiple push notifications because we were sending
it in a loop of mentioned user list.

To fix this, we clone the post every time.

https://mattermost.atlassian.net/browse/MM-36894

```release-note
NONE
```

* Refactor markdown parsing

```release-note
NONE
```

app/notification_push.go

@@ -56,12 +56,6 @@ type PushNotification struct {
 func (a *App) sendPushNotificationSync(post *model.Post, user *model.User, channel *model.Channel, channelName string, senderName string,
 	explicitMention bool, channelWideMention bool, replyToThreadType string) *model.AppError {
 	cfg := a.Config()
-	message, err := utils.StripMarkdown(post.Message)
-	if err != nil {
-		mlog.Warn("Failed parse to markdown", mlog.String("post_id", post.Id), mlog.Err(err))
-	} else {
-		post.Message = message
-	}
 	msg, appErr := a.BuildPushNotificationMessage(
 		*cfg.EmailSettings.PushNotificationContents,
 		post,
@@ -585,6 +579,12 @@ func (a *App) buildFullPushNotificationMessage(contentsConfig string, post *mode
 	}
 
 	postMessage := post.Message
+	stripped, err := utils.StripMarkdown(postMessage)
+	if err != nil {
+		mlog.Warn("Failed parse to markdown", mlog.String("post_id", post.Id), mlog.Err(err))
+	} else {
+		postMessage = stripped
+	}
 	for _, attachment := range post.Attachments() {
 		if attachment.Fallback != "" {
 			postMessage += "\n" + attachment.Fallback