Strong static typing and defensive programming

Codebase Refactoring Task: Type Safety & Defensive Programming

Execute the following systematic improvements across the Quill codebase:

Phase 1 - Python Type Safety

• Scan all Python files and add comprehensive type hints (function parameters, return types, class attributes)
• Use typing module types (Optional, Union, List, Dict, etc.) where appropriate
• Ensure all functions/methods have complete type annotations
• Run full test suite to verify no regressions

Phase 2 - TypeScript Type Safety

• Review all TypeScript files for any types and replace with specific types
• Add explicit return type annotations to all functions
• Ensure strict TypeScript config compliance (strict: true)
• Run full test suite to verify no regressions

Phase 3 - Python Defensive Programming

• Add input validation at the start of every function/method
• Validate argument types, ranges, null checks, and constraints
• Raise appropriate exceptions (ValueError, TypeError) with descriptive messages
• Run full test suite to verify no regressions

Phase 4 - TypeScript Defensive Programming

• Add runtime input validation at function entry points
• Implement type guards and null checks where needed
• Throw appropriate errors with clear messages
• Run full test suite to verify no regressions

Critical Rules

• Test after EACH phase before proceeding
• Do not modify business logic
• Preserve existing functionality exactly
• Document any assumptions made during validation