A simplified unit test cluster with shared DI and routine setup

[alexdresko]

@ReubenBond since you created the InProcessTestCluster, would you mind looking at the solution I created for #8790?

In short, we need the ability to set up our tests using WebApplication.CreateBuilder() as the starting point, as that allows for a more complete setup of the root builder. It more closely resembles the standard setup for an application using Orleans (builder.UseOrleans(...)), and maintains a shared DI container between the tests and Orleans, all while (theoretically) inheriting the performance benefits of InProcessTestCluster.

I would like to get this merged into the main repo due to the requirements for [assembly: InternalsVisibleTo("Orleans.TestingHost.HostBuilder")]. I wanted to get some feedback before heading down that path, though.

Please, and thank you for your time.

See the working example here: https://github.com/alexdresko/orleans/blob/alex-2/test/HostBuilderTestCluster.Tests/HostBuilderTestClusterTests.cs

[Fact]
public async Task HostBuilderCluster_RunsGrain()
{
    // Create a standard WebApplicationBuilder
    var builder = WebApplication.CreateBuilder();
    
    // Register your services - this is the SAME container Orleans will use
    builder.Services.AddSingleton(new MySingletonService { Age = 42 });
    
    // Add Orleans test cluster to the same builder
    var orleansBuilder = builder.AddHostBuilderTestCluster();
    orleansBuilder.Options.ConfigureFileLogging = false;

    var app = orleansBuilder.Build();
    await using var cluster = await orleansBuilder.DeployAsync();

    // Call grain - it uses the SAME singleton instance
    var grain = cluster.Client.GetGrain<IEchoGrain>(0);
    var response = await grain.Echo(13);
    Assert.Equal(13, response);

    // Test code can access the SAME singleton
    var service = app.Services.GetRequiredService<MySingletonService>();
    Assert.Equal(13, service.Age); // Value was modified by grain!
}

The cluster itself is at https://github.com/alexdresko/orleans/tree/alex-2/src/Orleans.TestingHost.HostBuilder

[alexdresko]

I should mention that some of the code is AI generated. I started with how I wanted the tests to look, and asked it to create a starting point based on InProcessTestCluster. If the implementation is way off base, let me know and I'll dig in. I'm just trying to get the ball rolling, and I think this is a good start if anything.

[alexdresko]

Random bump because I still really really need this and it seems like many others would as well.