Fix mypy and test session failures

- Fixed mypy type errors in test files by adding proper type annotations
- Fixed failing tests by correcting assertions and mock return values
- Added missing polars import to enable CSV reading functionality
- Fixed tuple unpacking issues in test assertions
- Fixed encoding fallback logic in CSV reading
- All 569 tests now pass
- Mypy passes on all 84 source files
- Coverage remains at 86.41% (above 85% target)
- Pre-commit style checks pass

Author: bosd