This closes#10513, thanks to @bastelfreak for the report.
GET /status/leader returns an IPv6 host with square brackets
around the IP address as expected, but the rpcAddr property
on the agent model does not.
This fixes rpcAddr, updates the Mirage /status/leader mock
to properly format an IPv6 host, and changes the agent
factory to sometimes produce IPv6 addresses.
I added a formatHost utility function to centralise the
conditional square bracket-wrapping that would have
otherwise been further scattered around.
