fix: use build output directory for backup test temp root to avoid low free-space failures on Windows CI runners

BackupServiceTests rooted its temp directory under Path.GetTempPath(), which
on GitHub-hosted windows-latest runners resolves to the constrained system C:
drive. BackupService.CreateBackupAsync requires 5GiB free at the backup path
before starting, and the C: drive's free temp space can dip below that,
failing CreateBackupAsync_WithCorruptKeyframeDataRow_SkipsRowAndCompletesBackup
even though the fix itself is correct. Rooting the test directory under
AppContext.BaseDirectory keeps it on the same (much larger) drive as the repo
checkout on all platforms, without touching the real BackupService free-space
check.

tests/Jellyfin.Server.Implementations.Tests/FullSystemBackup/BackupServiceTests.cs

@@ -50,7 +50,11 @@ public sealed class BackupServiceTests : IDisposable
             ctx.Database.EnsureCreated();
         }
 
-        _testRoot = Path.Combine(Path.GetTempPath(), "jellyfin-backup-service-tests-" + Guid.NewGuid().ToString("N"));
+        // Use the test assembly's own output directory instead of Path.GetTempPath(). On GitHub-hosted
+        // windows-latest runners, the system temp directory lives on the constrained C: drive, which can have
+        // less than the 5GiB BackupService requires free, causing spurious failures. AppContext.BaseDirectory
+        // is under the repo checkout (the much larger D: drive on Windows runners) on all platforms.
+        _testRoot = Path.Combine(AppContext.BaseDirectory, "jellyfin-backup-service-tests-" + Guid.NewGuid().ToString("N"));
         _backupPath = Path.Combine(_testRoot, "Backup");
         _configurationDirectoryPath = Path.Combine(_testRoot, "Config");
         Directory.CreateDirectory(_backupPath);