Use sellable quantity for Firstrade stale sell values

[Pigbibi]

Summary

• Use sellable quantity and current quote to prevent stale market values from under-sizing Firstrade sell orders.
• Preserve max order notional caps while allowing target-zero liquidation to sell the full available quantity when the cap allows it.
• Add regression coverage for the SOXL stale-value case.

Test

• python -m pytest tests/test_execution_service.py -q