Implement a blocking get in `SignedStateNexus`

- `SignedStateNexus::getState` returns `null` if the get fails
- it would be nice to have a blocking get, so that a caller doesn't need to implement that functionality manually

Start with SignedStateNexus::getState and trace how its failed get returns null and how callers currently handle that result. Define the blocking behavior and verify that callers can use it without implementing the waiting logic themselves.

Written by the indexing model from the issue text.