Add tests exposing 4 critical bugs

[cergfix]

Summary

• Added 7 failing tests that demonstrate 4 critical bugs found in the codebase
• Tests are integrated into existing test files (test_create.py, test_nested.py, test_hasher.py)
• All 122 existing tests continue to pass; the 7 new tests fail, proving the bugs are valid

Bugs covered

1. generator.py:75 — Class object inserted into hash entries list
hash_entries = [MHLHashEntry] creates a list containing the MHLHashEntry class itself as the first element instead of an empty list. This causes the class to be appended as a bogus entry into every media hash created via append_multiple_format_file_hashes.

2. history.py:105 — TypeError crash in nested history ignore patterns
latest_ignore_pattern_from_nested_histories() iterates over history.latest_ignore_patterns() which returns None when a child history has no hash lists or no ignore spec, causing TypeError: 'NoneType' object is not iterable.

3. history.py:370 — Early return aborts all reference resolution
_resolve_hash_list_references() uses return instead of continue when a reference path is not found in child_history_mappings, causing the method to abort processing all remaining references across all hash lists.

4. hasher.py:250 — Type assigned instead of dict in AggregateHasher
hasher_lookup = Dict[str, str] assigns the typing.Dict type object instead of creating an empty dict, crashing with TypeError on item assignment when AggregateHasher.__init__ is called.

Test plan

• All 122 existing tests pass
• All 7 new bug tests fail, confirming the bugs are real
• Fix the 4 bugs and verify all 129 tests pass