Add _accountRoutes ConcurrentDictionary to RouteManager with full
CRUD API: RegisterAccountRoute, UnregisterAccountRoute,
GetDedicatedAccountRoute, HasDedicatedRoute,
GetAccountsWithDedicatedRoutes, and DedicatedRouteCount property.
Update GetRouteForAccount to check dedicated routes before falling
back to pool-based selection. Add 10 unit tests in AccountRouteTests.
