fix: use equity field for balance in Unified Account API

[ezname]

Fix balance handling for Bybit Unified Account API

Issue

The current implementation uses availableToWithdraw field from the Bybit API response to get account balance. However, with the Unified Account API, this field is often returned as an empty string, causing the code to fail when trying to convert it to float.

Fix

Modified get_asset_balance method to use the equity field instead of availableToWithdraw. The equity field is consistently populated with the correct balance value in the Unified Account API response.

Changes

• Updated get_asset_balance method in bybit_store.py to use equity instead of availableToWithdraw
• Improved code formatting for better readability

Testing

Tested with Bybit Unified Account using both spot and linear perpetual markets. The fix allows the code to properly retrieve account balances and proceed with trading operations.