Fix test failures for serialization changes

- Update test_invalid_input_types to test actual serialization failure
- Update test_interpolate_only_unsupported_type_error with proper __repr__ method
- Update test_interpolate_invalid_type_validation to test serialization vs rejection
- Update test_interpolate_custom_object_validation to distinguish serializable vs unserializable objects
- All tests now align with new behavior where unsupported types are serialized instead of rejected

Co-Authored-By: João <joao@crewai.com>

tests/utilities/test_string_utils.py

@@ -91,16 +91,19 @@ class TestInterpolateOnly:
         assert "name" in str(excinfo.value)
 
     def test_invalid_input_types(self):
-        """Test that an error is raised with invalid input types."""
+        """Test that an error is raised when serialization fails."""
+        class UnserializableObject:
+            def __str__(self):
+                raise Exception("Cannot convert to string")
+            def __repr__(self):
+                raise Exception("Cannot convert to string")
+        
         template = "Hello, {name}!"
-        # Using Any for this test since we're intentionally testing an invalid type
-        inputs: Dict[str, Any] = {"name": object()}  # Object is not a valid input type
+        inputs: Dict[str, Any] = {"name": UnserializableObject()}
 
-        with pytest.raises(ValueError) as excinfo:
+        with pytest.raises(ValueError, match="Unable to serialize UnserializableObject"):
             interpolate_only(template, inputs)
 
-        assert "unsupported type" in str(excinfo.value).lower()
-
     def test_empty_input_string(self):
         """Test handling of empty or None input string."""
         inputs: Dict[str, Union[str, int, float, Dict[str, Any], List[Any]]] = {
@@ -223,6 +226,8 @@ class TestInterpolateOnly:
         class CustomObject:
             def __str__(self):
                 raise Exception("Cannot serialize")
+            def __repr__(self):
+                raise Exception("Cannot serialize")
         
         with pytest.raises(ValueError, match="Unable to serialize CustomObject"):
             interpolate_only("Value: {obj}", {"obj": CustomObject()})