Parse OKX spot swap errors into friendly Chinese instead of raw JSON.

Map sCode 51008 to a clear insufficient-balance message for the currency exchange UI.

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

tests/test_okx_spot_swap.py

@@ -30,6 +30,16 @@ class TestOkxSpotSwap(unittest.TestCase):
         self.assertEqual(body["tgtCcy"], "base_ccy")
         self.assertEqual(body["side"], "sell")
 
+    def test_insufficient_balance_returns_chinese_message(self):
+        ex = MagicMock()
+        ex.private_post_trade_order.side_effect = Exception(
+            'okx {"code":"1","data":[{"sCode":"51008","sMsg":"Order failed. Your available USDT balance is insufficient."}]}'
+        )
+        result = spot_market_swap_usdt_usdc(ex, direction="usdt_to_usdc", amount=20)
+        self.assertFalse(result["ok"])
+        self.assertEqual(result["msg"], "资金账户 USDT 可用余额不足")
+        self.assertNotIn("{", result["msg"])
+
 
 if __name__ == "__main__":
     unittest.main()