#334 - Add more type annotations

[reckart]

What's in the PR

• Replace builtin any with typing.Any in Dict type annotations across json.py
• Correct return types for properties that can be None: sofaString, sofa_mime, sofa_uri, document_language (cas.py), get_covered_text (typesystem.py)
• Fix invalid type syntax for _parse_primitive_array parameter (change from [list, str] to Union[list, str])
• Update _parse_primitive_array return type to Union[list, bytes] to match actual return paths
• Update _serialize_ref return type to Optional[int] (can return None)
• Remove incorrect -> str return annotation from document_language setter (setters return None)
• Rename duplicate nested function parse_and_add to parse_and_add_entry in dict branch to fix redeclaration error
• Wrap dict_values in iter() for TypeSystem.get_types() to maintain Iterator contract

How to test manually

• No specific test procedure

Automatic testing

• PR includes unit tests

Documentation

• PR updates documentation