fix: serialize trading history query data

[willzhqiang]

Summary

• serialize pandas/numpy values returned by trading history query/export APIs into JSON-safe values
• accept dict user_param for query_data/export_data while keeping legacy empty string compatibility
• add focused regression tests for DataFrame serialization and user_param normalization

Why

The trading history query/export endpoints can return pandas objects or require user_param to be passed as a dict to xtquant. Before this change, those responses could fail JSON serialization or pass an empty string where the underlying API expects a mapping.

Notes

This partially overlaps with #7 at the generic DataFrame serialization layer, but the scope here is trading query/export behavior plus user_param compatibility. If #7 lands first, this branch can be rebased and the shared helper changes adjusted.

Validation

• uv run --extra server --with pytest pytest -q tests/test_history_export.py
• 3 passed