test(jsonl): add jsonl_parser schema coverage and harden null usage

Add tests/test_jsonl_parser.py covering _parse_tool_result dispatch,
content helpers, user/assistant/system paths, file activity, parse_session
integration, quick_session_info, and malformed entries. Treat non-dict
message.usage in _process_assistant as empty to avoid crashes on null usage.

Author: clean6378-max-it