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

tests/unit/filter_plugins/test_get_service_name.py

@@ -0,0 +1,39 @@
+import unittest
+from filter_plugins import get_service_name
+
+class TestGetServiceName(unittest.TestCase):
+    def test_normal_service(self):
+        # Expect a dot between id and software name
+        self.assertEqual(
+            get_service_name.get_service_name("sys-ctl-cln-backups", "nginx"),
+            "sys-ctl-cln-backups.nginx.service"
+        )
+
+    def test_normal_service_custom_suffix(self):
+        self.assertEqual(
+            get_service_name.get_service_name("sys-ctl-cln-backups", "nginx", "timer"),
+            "sys-ctl-cln-backups.nginx.timer"
+        )
+
+    def test_with_at_suffix(self):
+        # If systemctl_id ends with '@', @ is moved behind software name
+        self.assertEqual(
+            get_service_name.get_service_name("sys-ctl-bkp@", "postgres"),
+            "sys-ctl-bkp.postgres@.service"
+        )
+
+    def test_with_at_and_custom_suffix(self):
+        self.assertEqual(
+            get_service_name.get_service_name("sys-ctl-bkp@", "postgres", "timer"),
+            "sys-ctl-bkp.postgres@.timer"
+        )
+
+    def test_case_is_lowered(self):
+        self.assertEqual(
+            get_service_name.get_service_name("Sys-CTL-BKP@", "Postgres", "SeRviCe"),
+            "sys-ctl-bkp.postgres@.service"
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()