fix: normalize trading data user_param

[doskoi]

Summary

• change export_data/query_data request models to default user_param to an object
• normalize omitted, None, or empty-string user_param to {} before calling xtquant
• add tests covering model defaults and manager forwarding

Why

xtquant export_data/query_data expects user_param to be a mapping. The previous default empty string can fail during BSON encoding with: encoder expected a mapping type but got: ''.

Tests

• uv run --with pytest --with pydantic python -m pytest tests/test_trading_data_user_param.py -q