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-subscriptions.json

@@ -0,0 +1,27 @@
+[
+ {
+  "startDate": "2026-08-21T21:03:46.816Z",
+  "endDate": "2026-11-21T21:03:46.816Z",
+  "paymentID": 0,
+  "interval": "month",
+  "paymentCancelled": false,
+  "plan": {
+   "id": "69561c587247b0ab681d4cd4",
+   "title": "LTT Supporter",
+   "description": "CONTENT\n\n- 2 new exclusives every week \u2014 including Why is WAN Late, behind-the-scenes, and bonus content\n\n- Access to exclusive livestreams\n\n- On-demand access to 400+ past exclusives\n\n- Save $16 by purchasing an annual subscription\n\n- Our gratitude for your support\n\nMERCH\n\n- Get access to purchase Floatplane-only merch on lttstore.com\n\n- Get access to quarterly limited-time drops\n\n- Enjoy early access to some sale events on lttstore.com",
+   "price": "8.00",
+   "priceYearly": "80.00",
+   "currency": "usd",
+   "creator": "59f94c0bdd241b70349eb72b",
+   "logo": null,
+   "featured": true,
+   "allowGrandfatheredAccess": false,
+   "discordServers": [],
+   "discordRoles": [],
+   "enabled": true,
+   "published": true,
+   "enabledGlobal": true
+  },
+  "creator": "59f94c0bdd241b70349eb72b"
+ }
+]