refactor: Moved business-logic from `ChatRoomService` into `ChatRoom`

refactor: Moved business-logic from `ChatRoomService` into `ChatRoom`

- Some essential business-logic -- the identification of mutated messages --
  was buried in `InMemoryChatRoomService`.
- This business-logic was moved into `ChatRoom`, becaus otherwise, it
  would have been necessary, to reproduce this logic in each and every
  new implementation of `ChatRoomService`, which would have been exhausting
  and errorprone.
- This allowed also cleaner code in `InMemoryChatRoomService`, that
  can focus on the persistence-logic.
- The implementation of `MessageMutationException` and the look of the
  accompanying problem-details hat to be refined accordingly.