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/TestEntities.cs

@@ -3,11 +3,6 @@ using Musify.Domain.ValueObjects;
 
 namespace Musify.Application.Tests.TestSupport
 {
-    /// <summary>
-    /// Builds fully-populated domain entities for tests, with sensible defaults for every
-    /// <c>required</c> member. Every factory always returns a valid instance — never null — so
-    /// callers never need a null check on what this class hands back.
-    /// </summary>
     public static class TestEntities
     {
         public static User User(