Skip processing policy server events through policy server

[turt2live]

This is to (primarily) allow a room to disable the policy server while it is down.

See https://github.com/matrix-org/matrix-spec-proposals/pull/4284/files#r2070860418 for related thoughts.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[anoadragon453]

CI failures are due to flakes, merging manually.