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/Fixtures/user-self.json

@@ -0,0 +1,24 @@
+{
+ "id": "000000000000000000000000",
+ "username": "MrLinuxGamer",
+ "email": "user@example.invalid",
+ "displayName": "",
+ "profileImage": {
+  "width": 512,
+  "height": 512,
+  "path": "https://pbs.floatplane.com/profile_images/default/user12.png",
+  "childImages": [
+   {
+    "width": 100,
+    "height": 100,
+    "path": "https://pbs.floatplane.com/profile_images/default/user12_100x100.png"
+   },
+   {
+    "width": 250,
+    "height": 250,
+    "path": "https://pbs.floatplane.com/profile_images/default/user12_250x250.png"
+   }
+  ]
+ },
+ "creators": []
+}