llama-cpp: add grafana annotations for inference requests

Poll /slots endpoint, create annotations when slots start processing,
close with token count when complete. Includes NixOS VM test with
mock llama-cpp and grafana servers. Dashboard annotation entry added.

tests/tests.nix

@@ -28,6 +28,9 @@ in
   # zfs scrub annotations test
   zfsScrubAnnotationsTest = handleTest ./zfs-scrub-annotations.nix;
 
+  # llama-cpp annotation service test
+  llamaCppAnnotationsTest = handleTest ./llama-cpp-annotations.nix;
+
   # ntfy alerts test
   ntfyAlertsTest = handleTest ./ntfy-alerts.nix;