fix: show success banner immediately on multisite wizard complete page (#880)

After all 4 install steps succeed, the JS auto-submits the form which
redirects to the complete step. Previously, section_complete() checked
is_multisite() to decide whether to show the success banner or manual
instructions. But is_multisite() returns false on the first load after
MULTISITE=true is written to wp-config.php, because the current PHP
process loaded before the constant existed (OPcache or same-request
timing). This caused users to see scary 'unable to configure' manual
instructions despite a fully successful automated install.

Fix: add a handle_install_complete() handler for the install step that
redirects to the complete step with result=success as a query parameter.
section_complete() already checks for this parameter, so the green
success banner now appears immediately without requiring a page refresh.

Author: superdav42

inc/admin-pages/class-multisite-setup-admin-page.php

@@ -147,6 +147,7 @@ public function get_sections() {
 				'description'  => __('Setting up your WordPress Multisite network...', 'multisite-ultimate'),
 				'next_label'   => Core_Installer::get_instance()->all_done() ? __('Begin Ultimate Multisite Setup →', 'ultimate-multisite') : __('Install', 'ultimate-multisite'),
 				'disable_next' => true,
+				'handler'      => [$this, 'handle_install_complete'],
 				'back'         => false,
 				'fields'       => [
 					'terms' => [
@@ -307,6 +308,27 @@ public function handle_configure(): void {
 		exit;
 	}
 
+	/**
+	 * Handles the install step form submission.
+	 *
+	 * The JS auto-submits the form after all AJAX installation steps succeed.
+	 * This handler redirects to the complete step with result=success so
+	 * that section_complete() can show the success banner immediately,
+	 * even when is_multisite() returns false because OPcache is serving
+	 * a stale wp-config.php that doesn't yet have the MULTISITE constant.
+	 *
+	 * @since 2.6.1
+	 * @return void
+	 */
+	public function handle_install_complete(): void {
+
+		$next_url = add_query_arg('result', 'success', $this->get_next_section_link());
+
+		wp_safe_redirect($next_url);
+
+		exit;
+	}
+
 	/**
 	 * Completion section view.
 	 *