fix: 限制单笔报单最大50手，防止以损定仓计算出超大委托

Co-authored-by: Cursor <cursoragent@cursor.com>

position_sizing.py

@@ -9,6 +9,9 @@ from contract_specs import get_contract_spec
 MODE_FIXED = "fixed"
 MODE_RISK = "risk"
 
+# 单笔报单手数上限（防止以损定仓在止损过近时算出超大手数）
+DEFAULT_MAX_ORDER_LOTS = 50
+
 
 def normalize_sizing_mode(raw: str) -> str:
     m = (raw or MODE_RISK).strip().lower()
@@ -63,8 +66,8 @@ def calc_lots_by_risk(
     if max_by_margin < 1:
         return None, "可用资金不足以覆盖 1 手保证金"
     lots = min(lots, max_by_margin)
-    if max_lots is not None:
-        lots = min(lots, int(max_lots))
+    cap_lots = max_lots if max_lots is not None else DEFAULT_MAX_ORDER_LOTS
+    lots = min(lots, cap_lots)
     return lots, None