improvement: throw explicit error when creating channel manually via forAddress()

[theorangeguo]

When creating a gRPC channel manually using forAddress(host, port) with a raw IP, the URI defaults to the discovery scheme because DiscoveryClientResolverFactory registers with a high priority (Priority=6).

This causes the DiscoveryClient to attempt resolving the IP address as a service name, leading to a misleading "No servers found" error.

This commit adds a validation check to:

1. Detect if the service name is a raw IP address.
2. Throw an explicit IllegalArgumentException with actionable advice.
3. Guide users to use .forTarget("dns:///...") instead of .forAddress() to bypass the high-priority discovery resolver.