Fix null likedBy parsing and DocumentContent model_dump key loss

[dhruv7539]

Summary

• make AnswerLikes.liked_by nullable so likedBy: null responses parse successfully
• update DocumentContent.serialize_model to preserve values when model_dump() uses field names internally
• add regression tests for both cases

Tests

• PYTHONPATH=src pytest -q tests/test_model_regressions.py

Closes #47
Closes #105