refactor: improve get_service_name suffix handling and handler usage

- Updated filter_plugins/get_service_name.py:
  * Default suffix handling: auto-select .service (no '@') or .timer (with '@')
  * Explicit False disables suffix entirely
  * Explicit string suffix still supported
- Updated sys-systemctl handler to use new filter instead of SYS_SERVICE_SUFFIX
- Extended unit tests to cover new suffix behavior

Ref: https://chat.openai.com/share/8c2de9e6-daa0-44dd-ae13-d7a7d8d8b6d9

tests/unit/filter_plugins/test_get_service_name.py

@@ -2,30 +2,32 @@ 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
+    def test_default_suffix_service(self):
         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
+    def test_default_suffix_timer(self):
         self.assertEqual(
             get_service_name.get_service_name("sys-ctl-bkp@", "postgres"),
-            "sys-ctl-bkp.postgres@.service"
+            "sys-ctl-bkp.postgres@.timer"
         )
 
-    def test_with_at_and_custom_suffix(self):
+    def test_explicit_custom_suffix(self):
         self.assertEqual(
-            get_service_name.get_service_name("sys-ctl-bkp@", "postgres", "timer"),
-            "sys-ctl-bkp.postgres@.timer"
+            get_service_name.get_service_name("sys-ctl-bkp@", "postgres", "special"),
+            "sys-ctl-bkp.postgres@.special"
+        )
+
+    def test_explicit_false_suffix(self):
+        self.assertEqual(
+            get_service_name.get_service_name("sys-ctl-bkp@", "postgres", False),
+            "sys-ctl-bkp.postgres@"
+        )
+        self.assertEqual(
+            get_service_name.get_service_name("sys-ctl-cln-backups", "nginx", False),
+            "sys-ctl-cln-backups.nginx"
         )
 
     def test_case_is_lowered(self):