MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/app/channel.go

@@ -2700,7 +2700,7 @@ func (a *App) MarkChannelAsUnreadFromPost(c request.CTX, postID string, userID s
 	if !collapsedThreadsSupported || !a.IsCRTEnabledForUser(c, userID) {
 		return a.markChannelAsUnreadFromPostCRTUnsupported(c, postID, userID)
 	}
-	post, err := a.GetSinglePost(postID, false)
+	post, err := a.GetSinglePost(c, postID, false)
 	if err != nil {
 		return nil, err
 	}
@@ -2727,7 +2727,7 @@ func (a *App) MarkChannelAsUnreadFromPost(c request.CTX, postID string, userID s
 }
 
 func (a *App) markChannelAsUnreadFromPostCRTUnsupported(c request.CTX, postID string, userID string) (*model.ChannelUnreadAt, *model.AppError) {
-	post, appErr := a.GetSinglePost(postID, false)
+	post, appErr := a.GetSinglePost(c, postID, false)
 	if appErr != nil {
 		return nil, appErr
 	}
@@ -2766,7 +2766,7 @@ func (a *App) markChannelAsUnreadFromPostCRTUnsupported(c request.CTX, postID st
 	//                          If there are replies with mentions below the marked reply in the thread, then sum the mentions for the threads mention badge.
 	// In CRT Unsupported Client: Channel is marked as unread and new messages line inserted above the marked post.
 	//                            Badge on channel sums mentions in all posts (root & replies) including and below the post that was marked unread.
-	rootPost, appErr := a.GetSinglePost(post.RootId, false)
+	rootPost, appErr := a.GetSinglePost(c, post.RootId, false)
 	if appErr != nil {
 		return nil, appErr
 	}