Refactor ServerTestHost into focused partial-class files

[Copilot]

ServerTestHost.cs had grown into a large mixed-responsibility file, making the server host flow harder to navigate. This change splits the existing partial type by concern so lifecycle, inbound RPC handling, request execution, and outbound messaging each live in a dedicated file.

• What changed

  • Kept ServerTestHost.cs focused on host lifecycle and core state:
    • fields
    • constructor
    • identity properties
    • initialization guard
    • InternalRunAsync
    • Dispose
    • unhandled exception hooks
  • Extracted inbound RPC processing to ServerTestHost.MessageLoop.cs:
    • HandleMessagesAsync
    • HandleNotificationAsync
    • HandleRequestAsync
    • CompleteRequest
    • RpcInvocationState
  • Extracted request execution and telemetry shaping to ServerTestHost.RequestExecution.cs:
    • HandleRequestCoreAsync
    • per-request execution pipeline
    • discovery/run metric builders
  • Extracted outbound RPC/message helpers to ServerTestHost.Messaging.cs:
    • responses/errors
    • test updates
    • telemetry updates
    • pushed server log messages

• Behavioral note

  • Preserves the existing ServerTestHost surface area and call paths while reducing file size and isolating responsibilities.
  • Corrects the cancellation warning path so warnings are emitted only when request cancellation itself throws.

• Example split

  // ServerTestHost.MessageLoop.cs
  private async Task HandleMessagesAsync(CancellationToken cancellationToken) { ... }
  
  // ServerTestHost.RequestExecution.cs
  private async Task<object> HandleRequestCoreAsync(RequestMessage message, RpcInvocationState rpcInvocationState, CancellationToken cancellationToken) { ... }
  
  // ServerTestHost.Messaging.cs
  private async Task SendMessageAsync(string method, object? @params, CancellationToken cancellationToken, bool checkServerExit = false, bool rethrowException = true) { ... }

[Copilot]

Pull request overview

This PR refactors ServerTestHost (Microsoft.Testing.Platform server-mode host) by splitting the existing partial type into multiple concern-focused files, keeping the lifecycle/core state in ServerTestHost.cs while moving message-loop/RPC handling, request execution, and outbound messaging into dedicated partial-class files. It also fixes the cancellation-warning logic so warnings are emitted only when request-cancellation itself throws.

Changes:

• Split inbound JSON-RPC reading/dispatch and request tracking into ServerTestHost.MessageLoop.cs.
• Split request execution + telemetry metric shaping into ServerTestHost.RequestExecution.cs.
• Split outbound JSON-RPC response/notification helpers into ServerTestHost.Messaging.cs, keeping ServerTestHost.cs focused on lifecycle and core state.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/Hosts/ServerTestHost.cs	Removes message-loop, request execution, and messaging helpers to keep the main file focused on lifecycle/core state.
src/Platform/Microsoft.Testing.Platform/Hosts/ServerTestHost.MessageLoop.cs	New partial file containing the main message loop, inbound notification/request handling, and per-request cancellation state.
src/Platform/Microsoft.Testing.Platform/Hosts/ServerTestHost.RequestExecution.cs	New partial file containing request execution pipeline and telemetry metric builders for discovery/run.
src/Platform/Microsoft.Testing.Platform/Hosts/ServerTestHost.Messaging.cs	New partial file containing outbound responses/errors and pushed notifications (test updates, telemetry, logs).

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1