convert labeler label values to lists in prefixed_label property

- Ensures all values in the `prefixed_label` dictionary are lists.
- Adds a unit test to validate the behavior for labeler rules.

logprep/processor/labeler/rule.py

@@ -62,7 +62,7 @@ def label(self) -> dict:
 
     @property
     def prefixed_label(self) -> dict:
-        return {f"label.{key}": value for key, value in self.label.items()}
+        return {f"label.{key}": list(value) for key, value in self.label.items()}
 
     def conforms_to_schema(self, schema: LabelingSchema) -> bool:
         """Check if labels are valid."""

tests/unit/processor/labeler/test_labeler_rule.py

@@ -4,6 +4,7 @@
 # pylint: disable=missing-function-docstring
 # pylint: disable=wrong-import-position
 from copy import deepcopy
+
 import pytest
 
 from logprep.filter.expression.filter_expression import StringFilterExpression
@@ -268,3 +269,13 @@ def test_complex_lucene_regex_does_not_match_returns_true_for_matching_document(
         assert not rule.matches({"applyrule": "UPloXXXX"})
         assert not rule.matches({"applyrule": "88888888"})
         assert not rule.matches({"applyrule": "UPlo$$7"})
+
+    def test_prefixed_label_property_is_a_dicts_with_only_list_values(self):
+        rule_definition = {
+            "filter": 'applyrule: "yes"',
+            "labeler": {"label": {"reporter": {"windows"}}},  # label is given as set
+        }
+        rule = LabelerRule._create_from_dict(rule_definition)
+        assert all(
+            isinstance(val, list) for val in rule.prefixed_label.values()
+        ), "prefixed_labels contain non-list values"