Refactor: fix code smells in RequestProcessor, ResponseBuilder, and Verify.NUnit

[Copilot]

Several code smells identified across the codebase: async void fire-and-forget with swallowed exceptions, inconsistent delegation patterns, dead-code assertions, verbose dictionary manipulation, unused variables, and a concrete type leaking through a public interface.

Changes

• RequestProcessor — async void HandleRequest → async Task; call site uses explicit _ = discard. async void silently drops exceptions; async Task surfaces them properly.

  // Before
  private static async void HandleRequest(...) { ... }
  HandleRequest(request, requestBody, respond, handler);
  
  // After
  private static async Task HandleRequest(...) { ... }
  _ = HandleRequest(request, requestBody, respond, handler);

• RequestWasCalled.Get — was calling AssertHandler directly; all other methods delegate through CustomVerb. Made Get consistent.

• RequestWasNotCalled.AssertHandler — removed the else branch that asserted handler Is.Null when handler == null; trivially true, never fires.

• ResponseBuilder.AddHeader — replaced ContainsKey + conditional Add/assign with the dictionary indexer, which handles both cases natively.

• RequestHandlerFactory.CreateHandler — removed string cleanedPath = path pass-through variable with no transformation.

• IHttpMockRepository — return types changed from concrete HttpServer to IHttpServer.

Original prompt

Review the code for code smells and suggest refactorings

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