Fixed the Signature Body and wrote test for it

src/Client/Credentials/Handler/SignatureBodyFormHandler.php

@@ -2,6 +2,7 @@
 
 namespace Vonage\Client\Credentials\Handler;
 
+use GuzzleHttp\Psr7\Utils;
 use Psr\Http\Message\RequestInterface;
 use Vonage\Client\Credentials\CredentialsInterface;
 use Vonage\Client\Credentials\SignatureSecret;
@@ -17,6 +18,7 @@ public function __invoke(RequestInterface $request, CredentialsInterface $creden
         $body = $request->getBody();
         $body->rewind();
         $content = $body->getContents();
+
         $params = [];
         parse_str($content, $params);
         $params['api_key'] = $credentialsArray['api_key'];
@@ -28,9 +30,8 @@ public function __invoke(RequestInterface $request, CredentialsInterface $creden
         );
 
         $params = $signature->getSignedParams();
-        $body->rewind();
-        $body->write(http_build_query($params, '', '&'));
 
-        return $request;
+        $newBody = Utils::streamFor(http_build_query($params, '', '&'));
+        return $request->withBody($newBody);
     }
 }

test/Client/Credentials/Handler/SignatureBodyFormHandlerTest.php

@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+namespace VonageTest\Client\Credentials\Handler;
+
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Utils;
+use Vonage\Client\Credentials\SignatureSecret;
+use VonageTest\VonageTestCase;
+use Vonage\Client\Credentials\Handler\SignatureBodyFormHandler;
+
+class SignatureBodyFormHandlerTest extends VonageTestCase
+{
+    public function testSignatureBodyFormHandler()
+    {
+        $initialBody = http_build_query(['param1' => 'value1', 'param2' => 'value2']);
+
+        $request = new Request(
+            'POST',
+            '/test',
+            ['Content-Type' => 'application/x-www-form-urlencoded'],
+            Utils::streamFor($initialBody)
+        );
+
+        $credentials = new SignatureSecret('secret', 'sha256');
+
+        $handler = new SignatureBodyFormHandler();
+        $authRequest = $handler($request, $credentials);
+
+        // Check the modified body
+        $authRequest->getBody()->rewind();
+        $newBody = $authRequest->getBody()->getContents();
+
+        parse_str($newBody, $params);
+
+        $this->assertArrayHasKey('api_key', $params);
+        $this->assertArrayHasKey('sig', $params); // assuming Signature adds 'sig'
+    }
+}