fix: preserve empty chat message fields

[lin-hongkuan]

Summary

• Preserve empty string and other falsy-but-valid chat message fields when normalizing messages.
• Continue dropping only fields whose value is None.

Why

An explicitly empty content field is valid input in some chat/tool-call flows, but _copy_messages() previously removed it because it filtered on truthiness.

Testing

• python -m pytest tests/test_client.py::test_client_chat_preserves_empty_message_content tests/test_client.py::test_client_chat -q
• git diff --check