future-private: add change margin mode request

src/Requests/ChangeMarginModeRequestContract.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Msr\LaravelBitunixApi\Requests;
+
+use Psr\Http\Message\ResponseInterface;
+
+interface ChangeMarginModeRequestContract
+{
+    /**
+     * Change margin mode for a trading pair
+     *
+     * @param string $symbol Trading pair (e.g., 'BTCUSDT')
+     * @param string $marginCoin Margin coin (e.g., 'USDT')
+     * @param string $marginMode Margin mode ('ISOLATION' or 'CROSS')
+     * @return ResponseInterface
+     */
+    public function changeMarginMode(string $symbol, string $marginCoin, string $marginMode): ResponseInterface;
+}