test: add network namespace test

Tests networkNamespacePath and networkNamespaceService options.
Creates a network namespace, runs a mock Servarr inside it, verifies
namespace isolation (mock unreachable from default ns), and confirms
the init service provisions resources through the namespace.

tests/default.nix

@@ -14,4 +14,5 @@
   delayed-start = import ./delayed-start.nix { inherit pkgs lib self; };
   jellyseerr = import ./jellyseerr.nix { inherit pkgs lib self; };
   naming = import ./naming.nix { inherit pkgs lib self; };
+  network-namespace = import ./network-namespace.nix { inherit pkgs lib self; };
 }