refactor: improve ManifestListReader test structure

- Introduce unified ManifestListReaderTestBase for common functionality
- Unify V1 and V2 test class inheritance and setup patterns
- Extract test data preparation into dedicated helper methods
- Add TestNonPartitionedManifests() helper for validation
- Standardize test naming: remove version prefixes, use consistent patterns
- Improve code organization and reduce duplication
- Maintain all existing test coverage while enhancing maintainability

Author: HeartLinked