Strip comments and dedupe normalized literals in backend tests

- Remove all // and /// comments from the backend test suite.
- Replace hardcoded normalized-form string literals (e.g. NormalizedTitle/
  NormalizedName duplicating a title/name in upper case) with a variable
  plus .ToUpperInvariant(), so the two values can't drift apart.

backend/Tests/Musify.Api.Tests/Extensions/ErrorOrHttpExtensionsTests.cs

@@ -8,8 +8,6 @@ namespace Musify.Api.Tests.Extensions
 {
     public sealed class ErrorOrHttpExtensionsTests
     {
-        // The concrete IResult types (NoContent, UnauthorizedHttpResult, ...) resolve logging/problem-
-        // details services from HttpContext.RequestServices when they execute, so it can't be empty.
         private static readonly IServiceProvider Services = new ServiceCollection().AddLogging().AddProblemDetails().BuildServiceProvider();
 
         private static async Task<(int StatusCode, string Body)> ExecuteAsync(IResult result)