The dashboard auth cookie name was hardcoded to the constant
DashboardAuthenticationDefaults.CookieName (MxGatewayDashboard). Browser
cookies are scoped by host+path but NOT by port, so two gateway instances
sharing a hostname would clobber each other's dashboard session under the
shared name.
Add DashboardOptions.CookieName (MxGateway:Dashboard:CookieName); null/blank
keeps the canonical default. Applied in the existing dashboard cookie
PostConfigure (runs after the inline AddCookie default, so it wins). Behaviour
is unchanged when unset. Adds a Tests case for the override.
Joseph Doherty
e57d864ab2