feat: add decision_service with unit tests

Service layer provides validation, delegation to repository, and
get_disliked_listing_ids for filtering. All 7 unit tests pass.

tests/unit/test_decision_service.py

@@ -0,0 +1,82 @@
+"""Unit tests for decision_service."""
+from unittest.mock import MagicMock
+import pytest
+
+from models.decision import ListingDecision
+from services import decision_service
+
+
+class TestSetDecision:
+    def test_set_liked(self) -> None:
+        repo = MagicMock()
+        repo.upsert_decision.return_value = ListingDecision(
+            id=1, user_id=1, listing_id=100, listing_type="RENT", decision="liked"
+        )
+        result = decision_service.set_decision(
+            repo, user_id=1, listing_id=100,
+            listing_type="RENT", decision="liked",
+        )
+        assert result.decision == "liked"
+        repo.upsert_decision.assert_called_once_with(
+            user_id=1, listing_id=100, listing_type="RENT", decision="liked"
+        )
+
+    def test_set_disliked(self) -> None:
+        repo = MagicMock()
+        repo.upsert_decision.return_value = ListingDecision(
+            id=1, user_id=1, listing_id=100, listing_type="RENT", decision="disliked"
+        )
+        result = decision_service.set_decision(
+            repo, user_id=1, listing_id=100,
+            listing_type="RENT", decision="disliked",
+        )
+        assert result.decision == "disliked"
+
+    def test_invalid_decision_raises(self) -> None:
+        repo = MagicMock()
+        with pytest.raises(ValueError, match="Invalid decision"):
+            decision_service.set_decision(
+                repo, user_id=1, listing_id=100,
+                listing_type="RENT", decision="maybe",
+            )
+
+
+class TestGetDecisions:
+    def test_returns_all_decisions(self) -> None:
+        repo = MagicMock()
+        repo.get_decisions_for_user.return_value = [
+            ListingDecision(
+                id=1, user_id=1, listing_id=100,
+                listing_type="RENT", decision="liked",
+            ),
+        ]
+        result = decision_service.get_decisions(repo, user_id=1)
+        assert len(result) == 1
+
+
+class TestClearDecision:
+    def test_clear_existing(self) -> None:
+        repo = MagicMock()
+        repo.delete_decision.return_value = True
+        result = decision_service.clear_decision(
+            repo, user_id=1, listing_id=100, listing_type="RENT"
+        )
+        assert result is True
+
+    def test_clear_nonexistent(self) -> None:
+        repo = MagicMock()
+        repo.delete_decision.return_value = False
+        result = decision_service.clear_decision(
+            repo, user_id=1, listing_id=100, listing_type="RENT"
+        )
+        assert result is False
+
+
+class TestGetDislikedListingIds:
+    def test_returns_disliked_set(self) -> None:
+        repo = MagicMock()
+        repo.get_disliked_listing_ids.return_value = {200, 300}
+        result = decision_service.get_disliked_listing_ids(
+            repo, user_id=1, listing_type="RENT"
+        )
+        assert result == {200, 300}