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/Mixes/GenerateMixesForUserCommandHandlerTests.cs

@@ -78,9 +78,6 @@ namespace Musify.Application.Tests.Mixes
         [Fact]
         public async Task Handle_YouTubeSourceUnavailable_StillCreatesMixesFromLocalCandidatesOnly()
         {
-            // Simulates IYouTubeMusicService.SearchSongsAsync as it behaves when the YouTube source is
-            // disabled (DisabledYouTubeMusicService, see Infrastructure): every call fails. Mix
-            // generation must degrade to local-only candidates instead of failing the whole command.
             youTubeMusicService
                 .SearchSongsAsync(Arg.Any<string>(), Arg.Any<string>(), Arg.Any<CancellationToken>())
                 .Returns(Error.Forbidden(description: "The YouTube Music source is disabled."));

backend/Tests/Musify.Application.Tests/Shared/TrackApplicationResponseTests.cs

@@ -26,8 +26,8 @@ namespace Musify.Application.Tests.Shared
         public void FromEntity_ExternalTrackWithArtists_JoinsThemInPositionOrder()
         {
             var track = TestEntities.ExternalTrack();
-            var second = new Artist { Name = "Second Artist", NormalizedName = "SECOND ARTIST", ExternalId = "second" };
-            var first = new Artist { Name = "First Artist", NormalizedName = "FIRST ARTIST", ExternalId = "first" };
+            var second = TestEntities.Artist("Second Artist", externalId: "second");
+            var first = TestEntities.Artist("First Artist", externalId: "first");
             track.TrackArtists =
             [
                 new TrackArtist { TrackId = track.Id, ArtistId = second.Id, Position = 1, Artist = second, Track = track },

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)

backend/Tests/Musify.Application.Tests/TestSupport/TestConfigurations.cs

@@ -2,10 +2,6 @@ using Musify.Application.Configuration;
 
 namespace Musify.Application.Tests.TestSupport
 {
-    /// <summary>
-    /// Configuration POCOs with every <c>required</c> member filled and the rest left at their
-    /// production defaults, so handler tests don't have to know which fields matter for them.
-    /// </summary>
     public static class TestConfigurations
     {
         public static ApplicationStorageConfiguration Storage(string bucket = "test-bucket") =>

backend/Tests/Musify.Application.Tests/TestSupport/TestDatabase.cs

@@ -5,25 +5,6 @@ using Musify.Domain.Entities;
 
 namespace Musify.Application.Tests.TestSupport
 {
-    /// <summary>
-    /// An <see cref="IDatabase"/> backed by a private, in-memory SQLite database.
-    /// </summary>
-    /// <remarks>
-    /// <para>
-    /// <see cref="IDatabase"/> exposes raw <see cref="DbSet{TEntity}"/> properties, so handlers
-    /// compose real LINQ queries (<c>Where</c>, <c>Select</c>, <c>Include</c>, ...) against it.
-    /// Substituting <see cref="IDatabase"/> with a mock cannot express that — there is no
-    /// <see cref="IQueryable{T}"/> provider behind a mock. SQLite's in-memory mode gives us a real
-    /// relational engine instead: fast, no external dependency, and — unlike the EF Core InMemory
-    /// provider — it actually supports transactions, which two handlers rely on.
-    /// </para>
-    /// <para>
-    /// The model mirrors the parts of <c>Musify.Infrastructure.Persistence.Database</c> that affect
-    /// query shape (owned types, table-per-type inheritance, unique indexes). MassTransit outbox/saga
-    /// state and Postgres-specific configuration are left out — the Application layer never touches
-    /// them.
-    /// </para>
-    /// </remarks>
     public sealed class TestDatabase : DbContext, IDatabase
     {
         private readonly SqliteConnection connection;
@@ -34,11 +15,6 @@ namespace Musify.Application.Tests.TestSupport
             this.connection = connection;
         }
 
-        /// <summary>
-        /// Creates a fresh, empty database with its own private connection. Call <see cref="Seed"/>
-        /// (via <see cref="Seeding.EntitySeedExtensions"/>) to populate it, then <see cref="DisposeAsync"/>
-        /// when the test is done.
-        /// </summary>
         public static async Task<TestDatabase> CreateAsync()
         {
             var connection = new SqliteConnection("Filename=:memory:");

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(