test: add unit tests for DeezerMetadataService and LocalLibraryService

- Add DeezerMetadataServiceTests with mocked HTTP responses
- Add LocalLibraryServiceTests for song ID parsing and registration
- Configure xUnit test project with Moq and MVC Testing packages

octo-fiesta.Tests/LocalLibraryServiceTests.cs

@@ -0,0 +1,155 @@
+using octo_fiesta.Services;
+using octo_fiesta.Models;
+using Microsoft.Extensions.Configuration;
+
+namespace octo_fiesta.Tests;
+
+public class LocalLibraryServiceTests : IDisposable
+{
+    private readonly LocalLibraryService _service;
+    private readonly string _testDownloadPath;
+
+    public LocalLibraryServiceTests()
+    {
+        _testDownloadPath = Path.Combine(Path.GetTempPath(), "octo-fiesta-tests-" + Guid.NewGuid());
+        Directory.CreateDirectory(_testDownloadPath);
+
+        var configuration = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["Library:DownloadPath"] = _testDownloadPath
+            })
+            .Build();
+
+        _service = new LocalLibraryService(configuration);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_testDownloadPath))
+        {
+            Directory.Delete(_testDownloadPath, true);
+        }
+    }
+
+    [Fact]
+    public void ParseSongId_WithExternalId_ReturnsCorrectParts()
+    {
+        // Act
+        var (isExternal, provider, externalId) = _service.ParseSongId("ext-deezer-123456");
+
+        // Assert
+        Assert.True(isExternal);
+        Assert.Equal("deezer", provider);
+        Assert.Equal("123456", externalId);
+    }
+
+    [Fact]
+    public void ParseSongId_WithLocalId_ReturnsNotExternal()
+    {
+        // Act
+        var (isExternal, provider, externalId) = _service.ParseSongId("local-789");
+
+        // Assert
+        Assert.False(isExternal);
+        Assert.Null(provider);
+        Assert.Null(externalId);
+    }
+
+    [Fact]
+    public void ParseSongId_WithNumericId_ReturnsNotExternal()
+    {
+        // Act
+        var (isExternal, provider, externalId) = _service.ParseSongId("12345");
+
+        // Assert
+        Assert.False(isExternal);
+        Assert.Null(provider);
+        Assert.Null(externalId);
+    }
+
+    [Fact]
+    public async Task GetLocalPathForExternalSongAsync_WhenNotRegistered_ReturnsNull()
+    {
+        // Act
+        var result = await _service.GetLocalPathForExternalSongAsync("deezer", "nonexistent");
+
+        // Assert
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task RegisterDownloadedSongAsync_ThenGetLocalPath_ReturnsPath()
+    {
+        // Arrange
+        var song = new Song
+        {
+            Id = "ext-deezer-123456",
+            Title = "Test Song",
+            Artist = "Test Artist",
+            Album = "Test Album",
+            ExternalProvider = "deezer",
+            ExternalId = "123456"
+        };
+        var localPath = Path.Combine(_testDownloadPath, "test-song.mp3");
+        
+        // Create the file
+        await File.WriteAllTextAsync(localPath, "fake audio content");
+
+        // Act
+        await _service.RegisterDownloadedSongAsync(song, localPath);
+        var result = await _service.GetLocalPathForExternalSongAsync("deezer", "123456");
+
+        // Assert
+        Assert.Equal(localPath, result);
+    }
+
+    [Fact]
+    public async Task GetLocalPathForExternalSongAsync_WhenFileDeleted_ReturnsNull()
+    {
+        // Arrange
+        var song = new Song
+        {
+            Id = "ext-deezer-999999",
+            Title = "Deleted Song",
+            Artist = "Test Artist",
+            Album = "Test Album",
+            ExternalProvider = "deezer",
+            ExternalId = "999999"
+        };
+        var localPath = Path.Combine(_testDownloadPath, "deleted-song.mp3");
+        
+        // Create and then delete the file
+        await File.WriteAllTextAsync(localPath, "fake audio content");
+        await _service.RegisterDownloadedSongAsync(song, localPath);
+        File.Delete(localPath);
+
+        // Act
+        var result = await _service.GetLocalPathForExternalSongAsync("deezer", "999999");
+
+        // Assert
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task RegisterDownloadedSongAsync_WithNullProvider_DoesNothing()
+    {
+        // Arrange
+        var song = new Song
+        {
+            Id = "local-123",
+            Title = "Local Song",
+            Artist = "Local Artist",
+            Album = "Local Album",
+            ExternalProvider = null,
+            ExternalId = null
+        };
+        var localPath = Path.Combine(_testDownloadPath, "local-song.mp3");
+
+        // Act - should not throw
+        await _service.RegisterDownloadedSongAsync(song, localPath);
+
+        // Assert - nothing to assert, just checking it doesn't throw
+        Assert.True(true);
+    }
+}