future-public: add set leverage request

tests/ChangeLeverageTest.php

@@ -0,0 +1,25 @@
+<?php
+
+use Msr\LaravelBitunixApi\Requests\ChangeLeverageRequestContract;
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+
+it('can change leverage successfully', function () {
+    $api = app(ChangeLeverageRequestContract::class);
+    // This test will make a real API call if credentials are valid
+    // For testing purposes, we'll just verify the method exists and can be called
+    expect(fn() => $api->changeLeverage('BTCUSDT', 'USDT', 12))
+        ->not->toThrow(Exception::class);
+});
+
+it('validates required parameters for change leverage', function () {
+    $api = app(ChangeLeverageRequestContract::class);
+
+    expect(fn() => $api->changeLeverage('BTCUSDT', 'USDT', 10))
+        ->not->toThrow(Exception::class)
+        ->and(fn() => $api->changeLeverage('BTCUSDT', 'USDT', 0))
+        ->not->toThrow(Exception::class);
+});
+