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-plain/README.md

@@ -7,7 +7,7 @@ This role manages WireGuard on a client system. It sets up essential services an
 ## Overview
 
 Optimized for client configurations, this role:
-- Deploys a systemd service (`set-mtu{{ SYS_SERVICE_SUFFIX }}`) and its associated script to set the MTU on specified network interfaces.
+- Deploys a systemd service and its associated script to set the MTU on specified network interfaces.
 - Uses a Jinja2 template to generate the `set-mtu.sh` script.
 - Ensures that the MTU is configured correctly before starting WireGuard with [wg-quick](https://www.wireguard.com/quickstart/).
 

roles/svc-net-wireguard-plain/templates/systemctl.service.j2

@@ -1,10 +1,10 @@
 [Unit]
 Description=set MTU 
-Before=wg-quick@wg0{{ SYS_SERVICE_SUFFIX }}
+Before=wg-quick@wg0.service
 
 [Service]
 Type=oneshot
 ExecStart=bash {{ systemctl_id | get_service_script_path('sh') }}
 
 [Install]
-RequiredBy=wg-quick@wg0{{ SYS_SERVICE_SUFFIX }}
+RequiredBy=wg-quick@wg0.service