Fix Bybit isolated balance sizing

tests/test_bybit_gateway.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from decimal import Decimal
 from typing import Any
 
 from llm_trade_bot.bybit_gateway import BybitGateway
@@ -7,9 +8,17 @@ from llm_trade_bot.config import BybitConfig, ScannerConfig
 
 
 class FakeBybitHTTP:
-    def __init__(self) -> None:
+    def __init__(self, wallet: dict[str, Any] | None = None) -> None:
+        self.wallet = wallet
         self.open_order_requests: list[dict[str, Any]] = []
 
+    def get_wallet_balance(self, **parameters: Any) -> dict[str, Any]:
+        assert parameters == {"accountType": "UNIFIED"}
+        return self.wallet or {"retCode": 0, "result": {"list": []}}
+
+    def get_positions(self, **parameters: Any) -> dict[str, Any]:
+        return {"retCode": 0, "result": {"list": []}}
+
     def get_open_orders(self, **parameters: Any) -> dict[str, Any]:
         self.open_order_requests.append(parameters)
         return {"retCode": 0, "result": {"list": []}}
@@ -26,3 +35,38 @@ async def test_open_orders_scopes_linear_requests_by_settle_coin() -> None:
         {"category": "spot", "limit": 50},
         {"category": "linear", "limit": 50, "settleCoin": "USDC"},
     ]
+
+
+async def test_account_snapshot_uses_quote_coin_balance_when_account_available_is_empty() -> None:
+    wallet = {
+        "retCode": 0,
+        "result": {
+            "list": [
+                {
+                    "totalEquity": "100000",
+                    "totalAvailableBalance": "",
+                    "totalMarginBalance": "100000",
+                    "coin": [
+                        {
+                            "coin": "USDT",
+                            "walletBalance": "50000",
+                            "availableToWithdraw": "",
+                            "totalPositionIM": "100",
+                            "totalOrderIM": "200",
+                            "locked": "300",
+                            "bonus": "400",
+                        },
+                        {"coin": "USDC", "walletBalance": "50000", "availableToWithdraw": ""},
+                    ],
+                }
+            ]
+        },
+    }
+    gateway = BybitGateway(BybitConfig("key", "secret", "USDC"), ScannerConfig(quote_coin="USDT"))
+    gateway._http = FakeBybitHTTP(wallet)  # type: ignore[assignment]
+
+    spot = await gateway.account_snapshot("spot")
+    linear = await gateway.account_snapshot("linear")
+
+    assert spot.available_balance == Decimal("49000")
+    assert linear.available_balance == Decimal("50000")