Fix LeafNodeManager.StartAsync to launch solicited connections for RemoteLeaves
(config-file-parsed remotes) in addition to the programmatic Remotes list, and
update ParseEndpoint to handle nats-leaf:// scheme URLs. Add LeafNodeFixture
that polls /leafz for connection readiness and three E2E tests covering
hub→leaf delivery, leaf→hub delivery, and subject-scoped propagation.
