fix: repair site_admins after enabling multisite

[chubes4]

Fixes #2511.

Problem

Playground's enableMultisite step leaves site_admins as an empty string rather than the array( $admin_login ) core's populate_network() writes. Probed from inside the PHPUnit sandbox:

sitemeta: [
  {"meta_key":"admin_email",   "meta_value":"admin@example.org"},
  {"meta_key":"admin_user_id", "meta_value":"1"},
  {"meta_key":"site_admins",   "meta_value":""}
]
users: ["admin"]

The install only partially succeeds — admin_user_id and admin_email are correct and the user exists.

Why an empty value is worse than a missing one

Core falls back to a sane default only when the option is absent:

return get_site_option( 'site_admins', array( 'admin' ) );

Because the row exists as '', the default never applies. The empty string reaches in_array():

TypeError: in_array(): Argument #2 ($haystack) must be of type array, string given
  at wp-includes/capabilities.php

This fatals is_super_admin(), grant_super_admin(), revoke_super_admin() and every manage_network_* capability check — so any multisite suite touching network capabilities dies inside core rather than failing usefully, which reads as harness breakage.

Fix

A runPHP repair immediately after enableMultisite, mirroring the existing enableMultisite + runPHP pattern already used in site-seed-multisite.ts.

Idempotent and conservative:

• writes only when the value is not already a non-empty array
• derives the login from admin_user_id rather than assuming 'admin', falling back to the lowest-ID user
• single-site recipes unchanged

Tests

tests/playground-multisite-site-admins.test.ts asserts the repair step exists, runs after enableMultisite, guards on is_array, derives from admin_user_id, and that single-site recipes carry no multisite steps.

Existing playground-runtime-multisite and site-seed-multisite suites pass; npm run build clean.

Found via

Extra-Chill/extrachill-network PR #217, which worked around it by setting \$GLOBALS['super_admins'] in the affected test. That workaround can be removed once this ships.