Fix close ladder completion when position is flat

[jackhai9]

Summary

• confirm the exact LONG or SHORT position through the current Binance position response before classifying close-ladder exhaustion
• end single and continuous close ladders normally when the requested direction is confirmed flat
• retain failures for unknown submission outcomes, symbol or mode changes, and unresolved dialogs
• add unit and source-contract coverage for flat completion and asynchronous readiness

Validation

• npm test (442 passed)
• npm run build:binance-orderbook-trade
• npm run check:binance-orderbook-trade
• git diff --check