fix(validators): validate regex patterns at init time

ContainsValidator and ExcludesValidator now pre-compile regex patterns
during initialisation and raise InvalidThresholdError if invalid.

tests/test_validators/test_constraint.py

@@ -263,6 +263,11 @@ class TestContainsValidator:
         with pytest.raises(InvalidThresholdError, match="cannot be empty"):
             ContainsValidator(patterns=[])
 
+    def test_contains_validator_raises_on_invalid_regex(self) -> None:
+        """Test that invalid regex pattern raises error at init time."""
+        with pytest.raises(InvalidThresholdError, match="Invalid regex"):
+            ContainsValidator(patterns=[r"[invalid"])
+
     def test_contains_factory_function(self) -> None:
         """Test the contains() factory function."""
         validator = contains(patterns=["test"], case_sensitive=True)
@@ -327,6 +332,11 @@ class TestExcludesValidator:
         with pytest.raises(InvalidThresholdError, match="cannot be empty"):
             ExcludesValidator(patterns=[])
 
+    def test_excludes_validator_raises_on_invalid_regex(self) -> None:
+        """Test that invalid regex pattern raises error at init time."""
+        with pytest.raises(InvalidThresholdError, match="Invalid regex"):
+            ExcludesValidator(patterns=[r"[invalid"])
+
     def test_excludes_factory_function(self) -> None:
         """Test the excludes() factory function."""
         validator = excludes(patterns=["test"], case_sensitive=True)