Add LongBridge fractional order API probe

[Pigbibi]

Summary

• add a skipped manual LongBridge API probe that exercises HK simulated-account fractional order submission
• verify sub-one quantities still hit LongBridge SubmittedQuantity validation while 1.5-share limit orders can be probed and cancelled
• expose the SDK fractional buying-power flag through the LongBridge estimate adapter

Validation

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

Notes

The new API probe is skipped unless LONGBRIDGE_API_PROBE=1 and LongPort credentials are present. It is intended for the HK simulated account workflow, not regular CI.