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.Application.Tests/TestSupport/HandlerTestBase.cs

@@ -4,12 +4,6 @@ using Xunit;
 
 namespace Musify.Application.Tests.TestSupport
 {
-    /// <summary>
-    /// Base class for handler tests: gives every test method its own empty, isolated
-    /// <see cref="TestDatabase"/> (xUnit creates a new instance of the test class per test method,
-    /// so there is no state to reset between tests) and a no-op logger factory for handlers that
-    /// take an <see cref="ILogger{TCategoryName}"/>.
-    /// </summary>
     public abstract class HandlerTestBase : IAsyncLifetime
     {
         private TestDatabase database = null!;
@@ -22,7 +16,6 @@ namespace Musify.Application.Tests.TestSupport
 
         protected static ILogger<T> NoOpLogger<T>() => NullLogger<T>.Instance;
 
-        /// <summary>Adds the entities and persists them, so handlers under test see committed data.</summary>
         protected async Task SeedAsync(params object[] entities)
         {
             foreach (var entity in entities)