Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/ERC2771ContextMock.sol

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.9;
+
+import "./ContextMock.sol";
+import "../metatx/ERC2771Context.sol";
+
+// By inheriting from ERC2771Context, Context's internal functions are overridden automatically
+contract ERC2771ContextMock is ContextMock, ERC2771Context {
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address trustedForwarder) ERC2771Context(trustedForwarder) {
+        emit Sender(_msgSender()); // _msgSender() should be accessible during construction
+    }
+
+    function _msgSender() internal view override(Context, ERC2771Context) returns (address) {
+        return ERC2771Context._msgSender();
+    }
+
+    function _msgData() internal view override(Context, ERC2771Context) returns (bytes calldata) {
+        return ERC2771Context._msgData();
+    }
+}