Refactor TestHostBuilder into focused partial modules

[Copilot]

TestHostBuilder.cs had grown into a large, multi-responsibility file that was hard to navigate and review. This change splits the implementation into focused partial class files while keeping the existing API and host-building behavior intact.

• What changed

  • Reduced TestHostBuilder.cs to the class declaration, shared state, BuildAsync, and a small internal build context used to coordinate the refactored flow.
  • Moved common service setup into TestHostBuilder.CommonServices.cs.
  • Moved mode selection and host construction logic into TestHostBuilder.Modes.cs.
  • Moved shared private helpers into TestHostBuilder.Utilities.cs.
  • Moved test framework construction into TestHostBuilder.Framework.cs.

• Resulting structure

  • TestHostBuilder.cs — thin orchestration entry point
  • TestHostBuilder.CommonServices.cs — shared service/configuration setup
  • TestHostBuilder.Modes.cs — tools, help/info, orchestrator, controller, and test-host mode dispatch
  • TestHostBuilder.Utilities.cs — shared private helpers
  • TestHostBuilder.Framework.cs — framework/message bus construction

• Why this split

  • Separates setup, dispatch, framework construction, and utilities into reviewable units.
  • Keeps the top-level flow readable without changing public/internal surface area.

• Example

  public async Task<IHost> BuildAsync(
      ApplicationLoggingState loggingState,
      TestApplicationOptions testApplicationOptions,
      IUnhandledExceptionsHandler unhandledExceptionsHandler,
      DateTimeOffset createBuilderStart)
  {
      ApplicationStateGuard.Ensure(TestFramework is not null);
      BuildContext context = await SetupCommonServicesAsync(
          loggingState,
          testApplicationOptions,
          unhandledExceptionsHandler,
          createBuilderStart).ConfigureAwait(false);
  
      return await BuildHostAsync(context).ConfigureAwait(false);
  }