Fix all mypy type checking errors

[jhw-db]

Achieve 100% mypy compliance across the codebase by fixing all type checking errors.

Current Issues:

• no-any-return errors
• attr-defined errors on Sequence types
• Missing type annotations
• Incorrect type hints

Fixes Required:

• Fix no-any-return by explicitly typing intermediate variables
• Fix attr-defined by converting Sequence to list before append
• Improve type hints across all contexts
• Add proper return type annotations

Benefits:

• Type safety
• Better IDE support
• Catch bugs at compile time