[copyberry]

Strengthen session file change approval coverage

Why

The notification helper buffers unrelated server requests, so the session-scoped
file change approval test could overlook an unexpected second approval request.

What changed

• Consume approval resolution and file change completion events directly and
  verify their request, thread, turn, and item identities.
• Fail if the second patch requests approval after the first patch was accepted
  for the session.
• Shut down the app server gracefully and assert that it exits successfully.