Address lint failure and code review feedback

- Fix unused excinfo variable in test_empty_inputs_dictionary
- Extract nested validate_type function to module-level _validate_input_type
- Add constants for supported types (SUPPORTED_PRIMITIVE_TYPES, SUPPORTED_CONTAINER_TYPES)
- Improve type hints for interpolate_only function
- Remove unused imports (List, Union)

All tests pass and lint checks are clean.

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

tests/utilities/test_string_utils.py

@@ -182,7 +182,7 @@ class TestInterpolateOnly:
         template = "Hello, {name}!"
         inputs: Dict[str, Any] = {}
 
-        with pytest.raises(ValueError) as excinfo:
+        with pytest.raises(ValueError):
             interpolate_only(template, inputs)