Tests: xUnit project with HAR fixtures (50 tests); fix relative delivery/HLS URLs resolving to file:// on Linux

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Jellyfin.Plugin.Floatplane.Tests/Fixture.cs

@@ -0,0 +1,14 @@
+using System.IO;
+using System.Text.Json;
+using Jellyfin.Plugin.Floatplane.Api.Models;
+
+namespace Jellyfin.Plugin.Floatplane.Tests;
+
+internal static class Fixture
+{
+    public static string Path(string name) => System.IO.Path.Combine(System.AppContext.BaseDirectory, "Fixtures", name);
+
+    public static string Text(string name) => File.ReadAllText(Path(name));
+
+    public static T Json<T>(string name) => JsonSerializer.Deserialize<T>(Text(name), FloatplaneJson.Options)!;
+}