refactor(StakeVault): introduce `stakedToken()` API (#36)

Because the `stakedToken` property is `immutable`, solhint recommends to
make it in all caps. This commit changes the property to adhere to that
rule and also makes the property private.

To access the `stakedToken` there's now a `stakedToken()` function on
the contract.

.gas-snapshot

@@ -1,17 +1,18 @@
 ExecuteAccountTest:testDeployment() (gas: 26400)
-ExecuteAccountTest:test_RevertWhen_InvalidLimitEpoch() (gas: 964423)
+ExecuteAccountTest:test_RevertWhen_InvalidLimitEpoch() (gas: 982104)
 LeaveTest:testDeployment() (gas: 26172)
-LeaveTest:test_RevertWhen_NoPendingMigration() (gas: 652901)
+LeaveTest:test_RevertWhen_NoPendingMigration() (gas: 670554)
 LeaveTest:test_RevertWhen_SenderIsNotVault() (gas: 10562)
 LockTest:testDeployment() (gas: 26400)
-LockTest:test_RevertWhen_DecreasingLockTime() (gas: 967353)
+LockTest:test_RevertWhen_DecreasingLockTime() (gas: 985034)
 LockTest:test_RevertWhen_SenderIsNotVault() (gas: 10607)
 MigrateTest:testDeployment() (gas: 26172)
-MigrateTest:test_RevertWhen_NoPendingMigration() (gas: 652740)
+MigrateTest:test_RevertWhen_NoPendingMigration() (gas: 670393)
 MigrateTest:test_RevertWhen_SenderIsNotVault() (gas: 10629)
 StakeManagerTest:testDeployment() (gas: 26172)
 StakeTest:testDeployment() (gas: 26172)
 StakeTest:test_RevertWhen_SenderIsNotVault() (gas: 10638)
+StakedTokenTest:testStakeToken() (gas: 7638)
 UnstakeTest:testDeployment() (gas: 26355)
-UnstakeTest:test_RevertWhen_FundsLocked() (gas: 963816)
+UnstakeTest:test_RevertWhen_FundsLocked() (gas: 981497)
 UnstakeTest:test_RevertWhen_SenderIsNotVault() (gas: 10609)

contracts/StakeVault.sol

@@ -16,18 +16,19 @@ contract StakeVault is Ownable {
     error StakeVault__MigrationNotAvailable();
 
     StakeManager private stakeManager;
-    ERC20 immutable stakedToken;
+
+    ERC20 private immutable STAKED_TOKEN;
 
     event Staked(address from, address to, uint256 _amount, uint256 time);
 
     constructor(address _owner, ERC20 _stakedToken, StakeManager _stakeManager) {
         _transferOwnership(_owner);
-        stakedToken = _stakedToken;
+        STAKED_TOKEN = _stakedToken;
         stakeManager = _stakeManager;
     }
 
     function stake(uint256 _amount, uint256 _time) external onlyOwner {
-        stakedToken.transferFrom(msg.sender, address(this), _amount);
+        STAKED_TOKEN.transferFrom(msg.sender, address(this), _amount);
         stakeManager.stake(_amount, _time);
 
         emit Staked(msg.sender, address(this), _amount, _time);
@@ -39,12 +40,12 @@ contract StakeVault is Ownable {
 
     function unstake(uint256 _amount) external onlyOwner {
         stakeManager.unstake(_amount);
-        stakedToken.transferFrom(address(this), msg.sender, _amount);
+        STAKED_TOKEN.transferFrom(address(this), msg.sender, _amount);
     }
 
     function leave() external onlyOwner {
         stakeManager.leave();
-        stakedToken.transferFrom(address(this), msg.sender, stakedToken.balanceOf(address(this)));
+        STAKED_TOKEN.transferFrom(address(this), msg.sender, STAKED_TOKEN.balanceOf(address(this)));
     }
 
     /**
@@ -55,4 +56,8 @@ contract StakeVault is Ownable {
         if (address(migrated) == address(0)) revert StakeVault__MigrationNotAvailable();
         stakeManager = migrated;
     }
+
+    function stakedToken() external view returns (ERC20) {
+        return STAKED_TOKEN;
+    }
 }

test/StakeVault.t.sol

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.19;
+
+import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+
+import { Test } from "forge-std/Test.sol";
+import { Deploy } from "../script/Deploy.s.sol";
+import { DeploymentConfig } from "../script/DeploymentConfig.s.sol";
+import { StakeManager } from "../contracts/StakeManager.sol";
+import { StakeVault } from "../contracts/StakeVault.sol";
+
+contract StakeVaultTest is Test {
+    StakeManager internal stakeManager;
+
+    DeploymentConfig internal deploymentConfig;
+
+    StakeVault internal stakeVault;
+
+    address internal deployer;
+
+    address internal testUser = makeAddr("testUser");
+
+    address internal stakeToken;
+
+    function setUp() public virtual {
+        Deploy deployment = new Deploy();
+        (stakeManager, deploymentConfig) = deployment.run();
+        (deployer, stakeToken) = deploymentConfig.activeNetworkConfig();
+
+        vm.prank(testUser);
+        stakeVault = new StakeVault(testUser, ERC20(stakeToken), stakeManager);
+    }
+}
+
+contract StakedTokenTest is StakeVaultTest {
+    function setUp() public override {
+        StakeVaultTest.setUp();
+    }
+
+    function testStakeToken() public {
+        assertEq(address(stakeVault.stakedToken()), stakeToken);
+    }
+}