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);
  }

[Copilot]

Pull request overview

This PR refactors the large TestHostBuilder.cs into multiple partial class files to improve readability and reviewability. Behavior, public/internal surface, and host-building flow are preserved; only code organization changes. A new private nested BuildContext class is introduced to thread shared state through the new helper methods.

Changes:

• Split TestHostBuilder into partials: CommonServices, Modes, Framework, Utilities, while reducing the main file to a thin BuildAsync orchestration entry point.
• Introduced an internal BuildContext state object (with mutable settable properties for values populated during setup) to pass coordination state between phases.
• Captured constructor parameters into private fields (_environment, _processHandler, _runtimeFeature) so they can be referenced from the other partial files.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.cs	Reduced to class declaration, fields, BuildAsync, and the nested BuildContext type.
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.CommonServices.cs	Extracted SetupCommonServicesAsync covering services, configuration, telemetry, validation, and early-exit handling.
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.Modes.cs	Extracted dispatch into Tools/Help/Info/Orchestrator/Controllers/TestHost (server vs console) flows.
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.Framework.cs	Extracted BuildTestFrameworkAsync and CreateConsoleTestHost.
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.Utilities.cs	Extracted shared helpers (telemetry metadata, banner display, named-pipe connect, registration helpers, activity completion).

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 0