test(contracts): add AddressAliasHelper coverage and fix SystemConfig flaky test

packages/contracts-bedrock/test/L1/SystemConfig.t.sol

@@ -866,7 +866,16 @@ contract SystemConfig_SetFeature_Test is SystemConfig_TestInit {
 contract SystemConfig_IsFeatureEnabled_Test is SystemConfig_TestInit {
     /// @notice Tests that `isFeatureEnabled` returns false for unset features.
     /// @param _feature The feature to check.
-    function testFuzz_isFeatureEnabled_unsetFeature_succeeds(bytes32 _feature) external view {
+    function testFuzz_isFeatureEnabled_unsetFeature_succeeds(bytes32 _feature) external {
+        vm.startPrank(address(systemConfig.proxyAdmin()));
+
+        // Normalize CUSTOM_GAS_TOKEN to avoid environment-dependent state
+        if (systemConfig.isFeatureEnabled(Features.CUSTOM_GAS_TOKEN)) {
+            systemConfig.setFeature(Features.CUSTOM_GAS_TOKEN, false);
+        }
+
+        vm.stopPrank();
+
         assertFalse(systemConfig.isFeatureEnabled(_feature));
     }
 

packages/contracts-bedrock/test/vendor/AddressAliasHelper.t.sol

@@ -4,6 +4,40 @@ pragma solidity 0.8.15;
 import { Test } from "forge-std/Test.sol";
 import { AddressAliasHelper } from "src/vendor/AddressAliasHelper.sol";
 
+/// @title AddressAliasHelper_ApplyL1ToL2Alias_Test
+/// @notice Tests for the `applyL1ToL2Alias` function.
+contract AddressAliasHelper_ApplyL1ToL2Alias_Test is Test {
+    uint160 constant OFFSET = uint160(0x1111000000000000000000000000000000001111);
+
+    /// @notice Tests that applyL1ToL2Alias correctly adds the offset to L1 address.
+    /// @param _l1Address The L1 address to apply the alias to.
+    function testFuzz_applyL1ToL2Alias_addsOffset_succeeds(address _l1Address) external pure {
+        address l2Address = AddressAliasHelper.applyL1ToL2Alias(_l1Address);
+        uint160 expected;
+        unchecked {
+            expected = uint160(_l1Address) + OFFSET;
+        }
+        assertEq(uint160(l2Address), expected);
+    }
+}
+
+/// @title AddressAliasHelper_UndoL1ToL2Alias_Test
+/// @notice Tests for the `undoL1ToL2Alias` function.
+contract AddressAliasHelper_UndoL1ToL2Alias_Test is Test {
+    uint160 constant OFFSET = uint160(0x1111000000000000000000000000000000001111);
+
+    /// @notice Tests that undoL1ToL2Alias correctly subtracts offset from L2 address.
+    /// @param _l2Address The L2 address to undo the alias from.
+    function testFuzz_undoL1ToL2Alias_subtractsOffset_succeeds(address _l2Address) external pure {
+        address l1Address = AddressAliasHelper.undoL1ToL2Alias(_l2Address);
+        uint160 expected;
+        unchecked {
+            expected = uint160(_l2Address) - OFFSET;
+        }
+        assertEq(uint160(l1Address), expected);
+    }
+}
+
 /// @title AddressAliasHelper_Uncategorized_Test
 /// @notice General tests that are not testing any function directly of the `AddressAliasHelper`
 ///         contract or are testing multiple functions at once.