fix: Convert benchmark test files to ES module syntax

- Convert gas-usage.test.js from CommonJS require() to ES module import
- Convert execution-time.test.js from CommonJS require() to ES module import
- Convert throughput.test.js from CommonJS require() to ES module import
- Required because contracts/package.json has "type": "module"

contracts/test/benchmarks/execution-time.test.js

@@ -1,4 +1,4 @@
-const { expect } = require("chai");
+import { expect } from "chai";
 
 describe("Execution Time Benchmarks", function () {
   describe("EscrowService", function () {

contracts/test/benchmarks/gas-usage.test.js

@@ -1,4 +1,4 @@
-const { expect } = require("chai");
+import { expect } from "chai";
 
 describe("Gas Usage Benchmarks", function () {
   describe("EscrowService", function () {

contracts/test/benchmarks/throughput.test.js

@@ -1,4 +1,4 @@
-const { expect } = require("chai");
+import { expect } from "chai";
 
 describe("Throughput Benchmarks", function () {
   describe("EscrowService", function () {