fix: add isVToken const to Venus token mocks

src/venus/IVenus.sol

@@ -23,6 +23,7 @@ interface IComptroller {
 }
 
 interface IVToken {
+    function isVToken() external view returns (bool);
     function mint(uint256 mintAmount) external returns (uint256);
     function redeem(uint256 redeemTokens) external returns (uint256);
     function redeemUnderlying(uint256 redeemAmount) external returns (uint256);

src/venus/MockVBNB.sol

@@ -6,6 +6,7 @@ import {IVBnb} from "src/venus/IVenus.sol";
 
 /// @notice Native ETH variant of VToken for Venus protocol.
 contract MockVBNB is ERC20, IVBnb {
+    bool public constant isVToken = true;
     uint256 public exchangeRateStored = 0.02e18;
     mapping(address => uint256) public borrowBalance;
     address public admin;

src/venus/MockVToken.sol

@@ -12,6 +12,7 @@ import {IVToken} from "src/venus/IVenus.sol";
 contract MockVToken is ERC20, IVToken {
     using SafeERC20 for IERC20;
 
+    bool public constant isVToken = true;
     address public override underlying;
 
     // Exchange rate scaled by 1e18. Initial: 0.02 underlying per vToken