The broker was refactored (file-based session persistence -> shared-cache
persistence; WebSocket handshake -> client-speaks-first), leaving 5 tests
failing against APIs that no longer exist. Update them to the current design:

- WebSocket tests now send op=register first, then expect event=registered
  (the server no longer greets before the client registers).
- Remote-node queue test simulates a session owned by another cluster node via
  a direct shared-cache entry (foreign broker_node_id, no local WebSocket), so
  send_request() correctly takes the queue slow path instead of a websocket
  send. The old approach (touch broker_node_id) no longer works because the
  cache always records the owning node.
- Replace the obsolete file-based persistence tests (_persist_sessions_locked /
  _load_persisted_sessions / _state_path, all removed) with cache-based
  persistence tests: cross-node session visibility and the offline tombstone.

Full file now passes 11/11 (was 6 passed / 5 failed).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>