test: add wu_create_site integration tests for subdomain slug sanitization

Adds two integration tests to Site_Functions_Extended_Test that verify
the subdomain slug sanitization introduced in GH#812 (PR #822) works
end-to-end at the wu_create_site() API level:

- test_create_site_sanitizes_subdomain_slug: verifies that a path with
  special characters (e.g. '/My Cool Site!/') produces a sanitized
  subdomain hostname containing 'my-cool-site' and does NOT fail with
  'invalid_site_path'.

- test_create_site_returns_error_for_empty_subdomain_slug: verifies that
  a path of only invalid characters (e.g. '/!!!/') returns WP_Error with
  'invalid_site_path', preventing malformed hostnames.

To make these tests runnable in the subdirectory test environment (where
is_subdomain_install() checks the SUBDOMAIN_INSTALL constant), a new
filter 'wu_is_subdomain_install' wraps the is_subdomain_install() call
in wu_create_site(). Tests override via add_filter('wu_is_subdomain_install',
'__return_true') to force the subdomain code path without redefining constants.

Resolves #826

Author: superdav42

inc/functions/site.php

@@ -249,7 +249,7 @@ function wu_create_site($site_data) {
 		: '';
 	$domain_supplied           = '' !== $normalized_input_domain && $normalized_input_domain !== $normalized_network_domain;
 
-	if (is_multisite() && is_subdomain_install() && $path_supplied && ! $domain_supplied) {
+	if (is_multisite() && apply_filters('wu_is_subdomain_install', is_subdomain_install()) && $path_supplied && ! $domain_supplied) {
 		$raw_slug             = trim((string) $site_data['path'], '/');
 		$slug                 = sanitize_title_with_dashes(wu_clean($raw_slug));
 

tests/WP_Ultimo/Functions/Site_Functions_Extended_Test.php

@@ -259,4 +259,64 @@ public function test_subdomain_slug_sanitization_handles_unicode(): void {
 			'Slug should only contain valid hostname characters.'
 		);
 	}
+
+	/**
+	 * Test wu_create_site sanitizes subdomain slug from path on subdomain installs.
+	 *
+	 * When a path is provided on a subdomain install without an explicit domain,
+	 * the slug derived from the path should be sanitized before building the
+	 * subdomain hostname.
+	 */
+	public function test_create_site_sanitizes_subdomain_slug(): void {
+
+		// Force subdomain install mode via the wu_is_subdomain_install filter.
+		add_filter('wu_is_subdomain_install', '__return_true');
+
+		$site = wu_create_site(
+			[
+				'title' => 'Sanitized Slug Test',
+				'path'  => '/My Cool Site!/',
+			]
+		);
+
+		remove_all_filters('wu_is_subdomain_install');
+
+		// The slug should be sanitized (lowercase, dashes, no special chars).
+		// It may succeed as a site or fail for other reasons, but should NOT
+		// fail with 'invalid_site_path'.
+		if (is_wp_error($site)) {
+			$this->assertNotEquals(
+				'invalid_site_path',
+				$site->get_error_code(),
+				'A valid-ish path should not produce invalid_site_path after sanitization.'
+			);
+		} else {
+			// If it succeeded, verify the domain contains the sanitized slug.
+			$this->assertStringContainsString('my-cool-site', $site->get_domain());
+		}
+	}
+
+	/**
+	 * Test wu_create_site returns WP_Error for empty slug on subdomain installs.
+	 *
+	 * When the path contains only invalid characters that sanitize to an empty
+	 * string, wu_create_site should return a WP_Error with 'invalid_site_path'.
+	 */
+	public function test_create_site_returns_error_for_empty_subdomain_slug(): void {
+
+		// Force subdomain install mode via the wu_is_subdomain_install filter.
+		add_filter('wu_is_subdomain_install', '__return_true');
+
+		$result = wu_create_site(
+			[
+				'title' => 'Empty Slug Test',
+				'path'  => '/!!!/',
+			]
+		);
+
+		remove_all_filters('wu_is_subdomain_install');
+
+		$this->assertWPError($result);
+		$this->assertEquals('invalid_site_path', $result->get_error_code());
+	}
 }