Allow LongBridge fractional sell orders

[Pigbibi]

Summary

• allow LongBridge sell orders with fractional quantities at or above 1 share
• keep local rejection for sub-1 quantities and fractional buy orders
• cover the BOXX fractional sell path seen in production

Tests

• PYTHONPATH=src /home/ubuntu/Projects/LongBridgePlatform/.venv/bin/python -m unittest tests.test_longbridge_execution -v
• PYTHONPATH=src /home/ubuntu/Projects/LongBridgePlatform/.venv/bin/python -m ruff check src/quant_platform_kit/longbridge/execution.py tests/test_longbridge_execution.py
• git diff --check