feat: add StdConfig::exists method to check if key exists

src/StdConfig.sol

@@ -321,6 +321,25 @@ contract StdConfig {
         return get(vm.getChainId(), key);
     }
 
+    /// @dev    Checks the existence of a variable for a given chain ID and key, and returns a boolean.
+    ///         Example: `bool hasKey = config.exists(1, "my_key");`
+    ///
+    /// @param chain_id The chain ID to check.
+    /// @param key The variable key name.
+    /// @return `bool` indicating whether a variable with the given key exists.
+    function exists(uint256 chain_id, string memory key) public view returns (bool) {
+        return _dataOf[chain_id][key].length > 0;
+    }
+
+    /// @dev    Checks existance of a variable for the current chain id and a given key, and returns a boolean.
+    ///         Example: `bool hasKey = config.exists("my_key");`
+    ///
+    /// @param key The variable key name.
+    /// @return `bool` indicating whether a variable with the given key exists.
+    function exists(string memory key) public view returns (bool) {
+        return exists(vm.getChainId(), key);
+    }
+
     /// @notice Returns the numerical chain ids for all configured chains.
     function getChainIds() public view returns (uint256[] memory) {
         string[] memory keys = _chainKeys;

test/Config.t.sol

@@ -125,6 +125,35 @@ contract ConfigTest is Test, Config {
         assertEq(vm.getChainId(), 10);
     }
 
+    function test_configExists() public {
+        _loadConfig("./test/fixtures/config.toml", false);
+
+        string[] memory keys = new string[](7);
+        keys[0] = "is_live";
+        keys[1] = "weth";
+        keys[2] = "word";
+        keys[3] = "number";
+        keys[4] = "signed_number";
+        keys[5] = "b";
+        keys[6] = "str";
+
+        // Read and assert RPC URL for Mainnet (chain ID 1)
+        assertEq(config.getRpcUrl(1), "https://reth-ethereum.ithaca.xyz/rpc");
+
+        for (uint256 i = 0; i < keys.length; ++i) {
+            assertTrue(config.exists(1, keys[i]));
+            assertFalse(config.exists(1, string.concat(keys[i], "_")));
+        }
+
+        // Assert RPC URL for Optimism (chain ID 10)
+        assertEq(config.getRpcUrl(10), "https://mainnet.optimism.io");
+
+        for (uint256 i = 0; i < keys.length; ++i) {
+            assertTrue(config.exists(10, keys[i]));
+            assertFalse(config.exists(10, string.concat(keys[i], "_")));
+        }
+    }
+
     function test_writeConfig() public {
         // Create a temporary copy of the config file to avoid modifying the original.
         string memory originalConfig = "./test/fixtures/config.toml";