Validate load-balancing policy name early in ManagedChannelBuilder.defaultLoadBalancingPolicy()

[shivaspeaks]

Modify defaultLoadBalancingPolicy(String) to validate the provided policy name against the LoadBalancerRegistry immediately when the setter is called. If the policy name is not found in the registry, the method should throw an exception. This change ensures that configuration errors, such as typos, are caught at the call site rather than during channel construction.

[manmohak07]

Hi @shivaspeaks, I'd like to work on this issue, and here's a fix that I'd like to suggest.

  @Override
  public ManagedChannelImplBuilder defaultLoadBalancingPolicy(String policy) {
    Preconditions.checkState(directServerAddress == null,
        "directServerAddress is set (%s), which forbids the use of load-balancing policy",
        directServerAddress);
    Preconditions.checkArgument(policy != null, "policy cannot be null");
    LoadBalancerProvider provider = LoadBalancerRegistry.getDefaultRegistry().getProvider(policy);
    Preconditions.checkArgument(provider != null, "policy %s not found", policy);
    this.defaultLbPolicy = provider.getPolicyName();
    return this;
  }

But I have a question wrt to this change.
In ManagedChannelImplBuilderTest we have two tests that use "magic_balancer" (unregistered) and will break, if the above is implemented. How do we want to modify the test cases?

[shivaspeaks]

This looks pretty normal to fail as "magic_balancer" is not registered. Either we should go with registering this fake lb provider or we can go with "pick_first". Registering Fake LB Provider is what I'd prefer in this case to make a clear case.