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/post_test.go

@@ -257,7 +257,7 @@ func TestAttachFilesToPost(t *testing.T) {
 		assert.Len(t, infos, 1)
 		assert.Equal(t, info2.Id, infos[0].Id)
 
-		updated, appErr := th.App.GetSinglePost(post.Id, false)
+		updated, appErr := th.App.GetSinglePost(th.Context, post.Id, false)
 		require.Nil(t, appErr)
 		assert.Len(t, updated.FileIds, 1)
 		assert.Contains(t, updated.FileIds, info2.Id)