test: add permanent failure test

Verifies the service enters failed state after exhausting all
StartLimitBurst retries when the API never becomes available.
Checks StartLimitIntervalSec/Burst configuration and confirms
repeated timeout messages appear in the journal.

tests/default.nix

@@ -15,4 +15,5 @@
   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; };
+  permanent-failure = import ./permanent-failure.nix { inherit pkgs lib self; };
 }