feat: add user_active_asset_data to Info

[manantlerio]

Closes #274

Adds Info.user_active_asset_data(user, name), which posts {"type": "activeAssetData", "user": ..., "coin": ...} to /info. It returns the user's current leverage for that coin ({"type": "cross" | "isolated", "value": int}, plus rawUsd when isolated), along with maxTradeSzs, availableToTrade and markPx.

It follows the existing Info methods: name is resolved through name_to_coin like l2_snapshot and funding_history, and the return value is typed with the existing ActiveAssetData TypedDict that the activeAssetData websocket subscription already uses.

Tests: added test_user_active_asset_data with a recorded VCR cassette, matching the other info tests. pytest passes (37 passed), and pre-commit passes (black, isort, flake8, pylint, bandit, mypy).

AI assistance was used in preparing this change.