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/user.go

@@ -2723,7 +2723,7 @@ func (a *App) UpdateThreadFollowForUserFromChannelAdd(c request.CTX, userID, tea
 		return model.NewAppError("UpdateThreadFollowForUserFromChannelAdd", "app.user.update_thread_follow_for_user.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 
-	post, appErr := a.GetSinglePost(threadID, false)
+	post, appErr := a.GetSinglePost(c, threadID, false)
 	if appErr != nil {
 		return appErr
 	}
@@ -2772,7 +2772,7 @@ func (a *App) UpdateThreadFollowForUserFromChannelAdd(c request.CTX, userID, tea
 }
 
 func (a *App) UpdateThreadReadForUserByPost(c request.CTX, currentSessionId, userID, teamID, threadID, postID string) (*model.ThreadResponse, *model.AppError) {
-	post, err := a.GetSinglePost(postID, false)
+	post, err := a.GetSinglePost(c, postID, false)
 	if err != nil {
 		return nil, err
 	}
@@ -2805,7 +2805,7 @@ func (a *App) UpdateThreadReadForUser(c request.CTX, currentSessionId, userID, t
 		return nil, model.NewAppError("UpdateThreadReadForUser", "app.user.update_thread_read_for_user.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
 	}
 
-	post, err := a.GetSinglePost(threadID, false)
+	post, err := a.GetSinglePost(c, threadID, false)
 	if err != nil {
 		return nil, err
 	}