Revert unverified LongBridge fractional sell policy

[Pigbibi]

Summary

• Revert the unverified LongBridge fractional sell allowance so the adapter remains whole-share-only for submitted orders.
• Add a unit test covering fractional sell rejection before the API call.
• Add an opt-in multi-symbol LongBridge order quantity API probe test for tonight's side/kind/symbol validation.

Notes

• Production evidence shows a BOXX fractional market sell was accepted, but we have not validated the full LongBridge matrix across side/order type/market session.
• Strategy math may continue to calculate fractional targets; this keeps LongBridge execution conservative until the API matrix is explicitly verified.

Tests

• PYTHONPATH=src /home/ubuntu/Projects/LongBridgePlatform/.venv/bin/python -m unittest tests.test_longbridge_execution tests.test_longbridge_order_quantity_api_probe -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 tests/test_longbridge_order_quantity_api_probe.py
• git diff --check