[network-controller] Allow switching active chain without waiting for lookup

When `setActiveChain` is called, the function does not resolve until the network lookup has completed. In some cases we just want tomorrow know when the NetworkController state was updated, and waiting for lookup is not desired.

We should allow the caller to not wait on lookup when switching the active chain (maybe by splitting this into two methods, or adding an option to control whether we wait).

Start at the NetworkController setActiveChain entry point and trace where the network lookup completes relative to the state update. Determine how callers can observe the state update without waiting for lookup while preserving the existing behavior, then add coverage for both waiting modes.

Written by the indexing model from the issue text.