test(semantic): add semantic similarity tests

tests/test_validators/test_metric.py

@@ -207,3 +207,77 @@ class TestLexicalValidator:
         validator = lexical(min_jaccard=0.5, min_overlap=0.6)
         assert isinstance(validator, LexicalValidator)
         assert validator.name == "lexical"
+
+
+# SemanticValidator tests - conditionally run if sentence-transformers is installed
+class TestSemanticValidator:
+    """Tests for SemanticValidator."""
+
+    @staticmethod
+    def _skip_if_no_transformers() -> None:
+        """Skip test if sentence-transformers is not installed."""
+        pytest.importorskip("sentence_transformers")
+
+    def test_semantic_validator_passes_when_score_meets_threshold(self) -> None:
+        """Test that validator passes when semantic similarity meets threshold."""
+        self._skip_if_no_transformers()
+        from veritext.validators.metric import SemanticValidator
+
+        validator = SemanticValidator(min_score=0.5)
+        context = ValidationContext(reference="the cat sat on the mat")
+        result = validator.check("the cat sat on the mat", context)
+
+        assert result.passed is True
+        assert result.name == "semantic"
+        assert result.actual >= 0.99  # Identical text
+        assert result.threshold == 0.5
+
+    def test_semantic_validator_fails_when_score_below_threshold(self) -> None:
+        """Test that validator fails when semantic similarity is below threshold."""
+        self._skip_if_no_transformers()
+        from veritext.validators.metric import SemanticValidator
+
+        validator = SemanticValidator(min_score=0.99)
+        context = ValidationContext(reference="the cat sat on the mat")
+        result = validator.check(
+            "quantum physics describes particle behaviour", context
+        )
+
+        assert result.passed is False
+        assert result.name == "semantic"
+        assert result.actual < 0.99
+        assert "below minimum" in result.message
+
+    def test_semantic_validator_raises_on_missing_reference(self) -> None:
+        """Test that validator raises when reference is missing."""
+        self._skip_if_no_transformers()
+        from veritext.validators.metric import SemanticValidator
+
+        validator = SemanticValidator(min_score=0.5)
+        context = ValidationContext()
+
+        with pytest.raises(ValidationError, match="requires reference text"):
+            validator.check("some text", context)
+
+    def test_semantic_validator_raises_on_invalid_min_score(self) -> None:
+        """Test that invalid min_score raises error without loading model."""
+        # This test doesn't need sentence-transformers since validation happens first
+        with pytest.raises(InvalidThresholdError, match=r"between 0\.0 and 1\.0"):
+            from veritext.validators.metric import SemanticValidator
+
+            SemanticValidator(min_score=1.5)
+
+        with pytest.raises(InvalidThresholdError, match=r"between 0\.0 and 1\.0"):
+            from veritext.validators.metric import SemanticValidator
+
+            SemanticValidator(min_score=-0.1)
+
+    def test_semantic_factory_function(self) -> None:
+        """Test the semantic() factory function."""
+        self._skip_if_no_transformers()
+        from veritext.validators import semantic
+        from veritext.validators.metric import SemanticValidator
+
+        validator = semantic(min_score=0.6)
+        assert isinstance(validator, SemanticValidator)
+        assert validator.name == "semantic"