fix: add tailnet smoke checks and move Tailscale operator to stable

Add a post-deploy smoke test that validates Tailscale DNS, proxy readiness,
reachability, and service responses for Rancher, Grafana, and Prometheus.
Move the operator to the stable Helm repo/version and align the baseline docs
with the current HA private-only architecture.

infrastructure/addons/tailscale-operator/helmrelease-tailscale-operator.yaml

@@ -9,7 +9,7 @@ spec:
   chart:
     spec:
       chart: tailscale-operator
-      version: 1.95.91
+      version: 1.96.5
       sourceRef:
         kind: HelmRepository
         name: tailscale

infrastructure/addons/tailscale-operator/helmrepository-tailscale.yaml

@@ -5,4 +5,4 @@ metadata:
   namespace: flux-system
 spec:
   interval: 1h
-  url: https://pkgs.tailscale.com/unstable/helmcharts
+  url: https://pkgs.tailscale.com/helmcharts