update lending contract and lending test case

contracts/ytLending/Lending.sol

@@ -182,6 +182,7 @@ contract Lending is
 
     /**
      * @notice 取出基础资产（如果余额不足会自动借款）
+     * @dev 如果用户余额不足，会自动借款，借款金额为 amount，借款利率为 borrowRate，借款期限为 borrowPeriod
      */
     function withdraw(uint256 amount) external override nonReentrant whenNotPaused {
         accrueInterest();
@@ -224,6 +225,7 @@ contract Lending is
 
     /**
      * @notice 存入抵押品
+     * @dev 由于不涉及债务计算，存入抵押品反而会让账户更安全，所以不用更新利息因子
      */
     function supplyCollateral(address asset, uint256 amount) external override nonReentrant whenNotPaused {
         AssetConfig memory config = assetConfigs[asset];
@@ -272,8 +274,6 @@ contract Lending is
         int104 oldPrincipal = user.principal;
         
         // 计算当前实际余额（含利息）
-        // 如果 principal >= 0（存款），使用 supplyIndex
-        // 如果 principal < 0（借款），使用 borrowIndex
         uint256 index = oldPrincipal >= 0 ? supplyIndex : borrowIndex;
         int256 oldBalance = LendingMath.principalToBalance(oldPrincipal, index);
         
@@ -370,13 +370,19 @@ contract Lending is
         // 计算偿还和供应金额
         (uint104 repayAmount, uint104 supplyAmount) = LendingMath.repayAndSupplyAmount(oldPrincipal, newPrincipal);
         
-        // 更新全局状态
-        // 储备金通过减少 totalBorrowBase 和增加 totalSupplyBase 来承担坏账
+        // 更新全局状态（储备金通过减少 totalBorrowBase 和增加 totalSupplyBase 来承担坏账）
         totalSupplyBase += supplyAmount;
         totalBorrowBase -= repayAmount;
         
-        // 计算协议支付的债务（坏账部分）
-        uint256 basePaidOut = uint256(newBalance - oldBalance);
+        // 计算协议承担的坏账部分
+        // 坏账 = 用户债务 - 抵押品价值（当抵押品不足时）
+        uint256 basePaidOut = 0;
+        if (int256(collateralInBase) < -oldBalance) {
+            // 抵押品不足以覆盖债务，差额由协议储备金承担
+            basePaidOut = uint256(-oldBalance) - collateralInBase;
+        }
+        // 如果 collateralInBase >= -oldBalance，说明抵押品足够，无坏账
+        
         uint256 valueOfBasePaidOut = (basePaidOut * basePrice) / baseScale;
         
         // 发射债务吸收事件
@@ -513,18 +519,6 @@ contract Lending is
         return totalValue;
     }
 
-    /**
-     * @notice 获取最小借款抵押率
-     */
-    function _getMinBorrowCollateralFactor() internal view returns (uint64) {
-        uint64 minFactor = type(uint64).max;
-        for (uint i = 0; i < assetList.length; i++) {
-            uint64 factor = assetConfigs[assetList[i]].borrowCollateralFactor;
-            if (factor < minFactor) minFactor = factor;
-        }
-        return minFactor;
-    }
-
     // ========== View Functions ==========
 
     function getBalance(address account) external view override returns (int256) {