Add Editor callbacks to performance critical context

`OnGUI` is called every frame, so should be considered performance critical context. It is usually intended for working in the Editor, but can still affect performance.

Also follow the callbacks for `EditorApplication.update`.

No files or tests are named. Start by locating the existing performance-critical context handling, then trace how OnGUI and EditorApplication.update callbacks are discovered; done means both callback sources are covered consistently with the current context rules and the relevant tests pass.

Written by the indexing model from the issue text.