Fix test bad practices and tighten weak assertions

- StorageServiceTests: replace out-param + manual dispose with IAsyncLifetime
- PictureServiceTests: Assert.True bound checks -> Assert.InRange
- CreateAlbumCommandHandlerTests: assert the specific NotFound error type
  instead of just IsError, matching what the test name promises
- CreateAlbumRequestValidatorTests: replace a boundary Theory that computed
  its expected result inline with explicit Facts against the real
  AlbumReleaseYear constants
- Remove unused using directives across a handful of production and test
  files found during the review

Full suite verified green: 250/250 tests passing, 0 build warnings.

backend/Tests/Musify.Application.Tests/Albums/CreateAlbumCommandHandlerTests.cs

@@ -1,3 +1,4 @@
+using ErrorOr;
 using Musify.Application.Albums;
 using Musify.Application.Tests.TestSupport;
 using Xunit;
@@ -37,7 +38,6 @@ public sealed class CreateAlbumCommandHandlerTests : HandlerTestBase
 
         var result = await CreateHandler().Handle(command, CancellationToken.None);
 
-        Assert.True(result.IsError);
-        Assert.Equal(ErrorOr.ErrorType.NotFound, result.FirstError.Type);
+        Assert.Equal(ErrorType.NotFound, result.FirstError.Type);
     }
 }

backend/Tests/Musify.Application.Tests/Mixes/GenerateMixesForUserCommandHandlerTests.cs

@@ -3,7 +3,6 @@ using Musify.Application.Contracts;
 using Musify.Application.Mixes;
 using Musify.Application.Tests.TestSupport;
 using Musify.Domain.Entities;
-using Musify.Domain.ValueObjects;
 using NSubstitute;
 using Xunit;
 
@@ -47,7 +46,7 @@ public sealed class GenerateMixesForUserCommandHandlerTests : HandlerTestBase
         youTubeMusicService
             .SearchSongsAsync(Arg.Any<string>(), Arg.Any<string>(), Arg.Any<CancellationToken>())
             .Returns(new YouTubeSearchResult(
-                [new YouTubeSongResult("yt-1", "YouTube Song", "Seed Artist", "Album", 200, "https://img/thumb.jpg", false, [])],
+                [new YouTubeSongResult("yt-1", "YouTube Song", "Seed Artist", "Album", 200, "https://img/thumb.jpg", IsExplicit: false, [] )],
                 ContinuationToken: string.Empty));
         youTubeMusicService.ResolveArtworkUrl(Arg.Any<string>()).Returns(callInfo => callInfo.Arg<string>());
 

backend/Tests/Musify.Application.Tests/PlayLists/AddYouTubeTrackToPlayListCommandHandlerTests.cs

@@ -5,7 +5,6 @@ using Musify.Application.PlayLists;
 using Musify.Application.Services;
 using Musify.Application.Tests.TestSupport;
 using Musify.Domain.Entities;
-using Musify.Domain.ValueObjects;
 using NSubstitute;
 using Xunit;
 

backend/Tests/Musify.Application.Tests/PlayLists/CreatePlayListCommandHandlerTests.cs

@@ -1,5 +1,4 @@
 using ErrorOr;
-using Musify.Application.Configuration;
 using Musify.Application.Contracts;
 using Musify.Application.PlayLists;
 using Musify.Application.Services;

backend/Tests/Musify.Application.Tests/Tracks/GetTrackCoverQueryHandlerTests.cs

@@ -1,7 +1,6 @@
 using ErrorOr;
 using Musify.Application.Tests.TestSupport;
 using Musify.Application.Tracks;
-using Musify.Domain.ValueObjects;
 using Xunit;
 
 namespace Musify.Application.Tests.Tracks;