fix: use dynamic quantity/price precision per symbol from exchange info

Hardcoded round(qty, 1) caused -1111 Precision errors for TRXUSDT and
DOGEUSDT (stepSize=1, requires integers). Now lazily loads quantityPrecision
and pricePrecision from Binance futures_exchange_info per symbol. SL/TP
prices also use symbol-specific precision instead of hardcoded 4 decimals.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_exchange.py

@@ -23,6 +23,8 @@ def client():
     c = BinanceFuturesClient.__new__(BinanceFuturesClient)
     c.config = config
     c.symbol = config.symbol
+    c._qty_precision = 1
+    c._price_precision = 4
     return c
 
 
@@ -39,6 +41,8 @@ def exchange():
     c.config = config
     c.symbol = config.symbol
     c.client = MagicMock()
+    c._qty_precision = 1
+    c._price_precision = 4
     return c