Add comprehensive tests for type utilities

[AliiiBenn]

Description

Add comprehensive unit tests for all type utilities.

Motivation

Currently, there are no dedicated tests for:

• Type utilities (Pick, Omit, Partial, Required, KeyOf)
• String operations (Uppercase, Lowercase, etc.)
• GetAnnotations/DropAnnotations

Existing tests focus on ORM and FastAPI use cases but don't cover the basic functionality.

Tasks

• Create tests/test_type_utilities.py for Pick, Omit, Partial, Required, KeyOf
• Create tests/test_string_operations.py for string type operations
• Add tests for GetAnnotations/DropAnnotations
• Ensure all new tests pass

Acceptance criteria

• Tests for all type utilities
• Tests cover edge cases
• All tests pass