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/plugin_api.go

@@ -704,7 +704,7 @@ func (api *PluginAPI) GetPostThread(postID string) (*model.PostList, *model.AppE
 }
 
 func (api *PluginAPI) GetPost(postID string) (*model.Post, *model.AppError) {
-	post, appErr := api.app.GetSinglePost(postID, false)
+	post, appErr := api.app.GetSinglePost(api.ctx, postID, false)
 	if post != nil {
 		post = post.ForPlugin()
 	}