Align opt-in reporters with the realigned reporter contracts

- OperationStreamEmitter no longer sets 'required' (manager derives it from
  the event type)
- Problem matcher diagnostics use the kind-tagged source union and the
  registered RUSH_EXTERNAL_TOOL_PROBLEM code with its template

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: TheLarkInn