fix(StakeManager): initial MP should not increase when locking

When an account calls `lock()` on the `StakeManager` it ultimately
called `_mintInitialMP()` which besides increasing the accounts
`currentMP`, it also increases the accounts `initialMP`.

This seems incorrect, as `initialMP` should only increase at first
stake.

Also, notice how inside `lock()` it always passes an `amount` of `0` to
to `_mintInitialMP()`, so it only ever uses one code path of that
function.

This commit adjust `lock()` such that it no longer uses
`_mintInitialMP()` and simply inlines the calculation of the new MP to be
minted based on the lock up time delta.

It also removes a test that claims that `initialMP` should increase on
`lock()`.

contracts/StakeManager.sol

@@ -222,9 +222,14 @@ contract StakeManager is Ownable {
         if (deltaTime < MIN_LOCKUP_PERIOD || deltaTime > MAX_LOCKUP_PERIOD) {
             revert StakeManager__InvalidLockTime();
         }
-        _mintInitialMP(account, _timeToIncrease, 0);
+
+        uint256 mpToMint = _getMPToMint(account.balance, _timeToIncrease);
+
         //update account storage
+        totalSupplyMP += mpToMint;
+        account.currentMP += mpToMint;
         account.lockUntil = lockUntil;
+        account.lastMint = block.timestamp;
     }
 
     /**

test/StakeManager.t.sol

@@ -412,25 +412,6 @@ contract LockTest is StakeManagerTest {
         vm.expectRevert(StakeManager.StakeManager__InvalidLockTime.selector);
         userVault.lock(minLockup - 1);
     }
-
-    function test_ShouldIncreaseInitialMP() public {
-        uint256 stakeAmount = 100;
-        uint256 lockTime = stakeManager.MAX_LOCKUP_PERIOD();
-        StakeVault userVault = _createStakingAccount(testUser, stakeAmount);
-        (, uint256 balance, uint256 initialMP, uint256 currentMP,,,) = stakeManager.accounts(address(userVault));
-        uint256 totalSupplyMPBefore = stakeManager.totalSupplyMP();
-
-        vm.startPrank(testUser);
-        userVault.lock(lockTime);
-
-        (, uint256 newBalance, uint256 newInitialMP, uint256 newCurrentMP,,,) =
-            stakeManager.accounts(address(userVault));
-        uint256 totalSupplyMPAfter = stakeManager.totalSupplyMP();
-        assertGt(totalSupplyMPAfter, totalSupplyMPBefore, "totalSupplyMP");
-        assertGt(newInitialMP, initialMP, "initialMP");
-        assertGt(newCurrentMP, currentMP, "currentMP");
-        assertEq(newBalance, balance, "balance");
-    }
 }
 
 contract LeaveTest is StakeManagerTest {