Add JSON endpoint for CAPTCHA rendering

To be used inside WP/WC registration form to construct URL for CAPTCHA page.

[MAILPOET-6325]

mailpoet/tests/integration/API/JSON/v1/CaptchaTest.php

@@ -0,0 +1,29 @@
+<?php declare(strict_types = 1);
+
+namespace integration\API\JSON\v1;
+
+use MailPoet\API\JSON\Response;
+use MailPoet\API\JSON\v1\Captcha;
+use MailPoet\Captcha\CaptchaSession;
+use MailPoet\Captcha\CaptchaUrlFactory;
+use MailPoet\Config\Populator;
+
+class CaptchaTest extends \MailPoetTest {
+  public function _before() {
+    $populator = $this->diContainer->get(Populator::class);
+    $populator->up();
+
+    parent::_before();
+  }
+
+  public function testItCanRenderCaptcha(): void {
+    $captchaSession = $this->diContainer->get(CaptchaSession::class);
+    $urlFactory = $this->diContainer->get(CaptchaUrlFactory::class);
+
+    $captcha = new Captcha($captchaSession, $urlFactory);
+    $response = $captcha->render();
+
+    verify($response->status)->equals(Response::REDIRECT);
+    verify($response->location)->stringContainsString('mailpoet_router&endpoint=captcha&action=render&data=');
+  }
+}