refactor(systemd-services): migrate SYS_SERVICE_SUFFIX usage to get_service_name filter

Replaced all hardcoded service name concatenations with the new get_service_name filter.
This ensures consistency, proper lowercase formatting, and correct handling of '@' suffixed units.

Added unittests for the filter (normal, custom suffix, '@'-units, and lowercase normalization).

Context: see ChatGPT discussion https://chatgpt.com/share/68a38beb-b9bc-800f-b7ed-cdd2b64b2604

roles/svc-net-wireguard-core/Administration.md

@@ -17,12 +17,12 @@
 ### Activate Configuration
 ```bash
   cp /path/to/wg0.conf /etc/wireguard/wg0.conf
-  systemctl enable wg-quick@wg0{{ SYS_SERVICE_SUFFIX }} --now
+  systemctl enable wg-quick@wg0.service --now
 ```
 
 ### Check status
 ```bash
-  systemctl status wg-quick@wg0{{ SYS_SERVICE_SUFFIX }}
+  systemctl status wg-quick@wg0.service
 ```
 
 ## Other Resources

roles/svc-net-wireguard-core/handlers/main.yml

@@ -1,6 +1,6 @@
 - name: "restart wireguard"
   systemd:
-    name: wg-quick@wg0{{ SYS_SERVICE_SUFFIX }}
+    name: wg-quick@wg0.service
     state: restarted
     enabled: yes
     daemon_reload: yes