Code Quality: Declare WP globals and initialize variables for PHPStan level 1

[huzaifaalmesbah]

Resolves PHPStan level 1 variable.undefined errors in four user/plugin admin files (98 of 564 project-wide level-1 errors). The project's enforced level (0) continues to pass.

• Declare @global docblocks + global statements for core globals ($wpdb, $current_user, $wp_roles, $_wp_admin_css_colors, $status, $page, $plugins, $user_ID, $usersearch, $blog_id).
• Initialize $redirect in wp-admin/users.php for the empty( $_REQUEST ) branch.
• Initialize $user_id, $old_user_data at the top of wp_insert_user() and $manage_url in _wp_privacy_send_request_confirmation_notification() so they're always defined.
• Bug fix: wp-admin/user-edit.php:166 referenced an undefined $user_login in the multisite signup-email update query; corrected to $user->user_login.

Files changed

File	Errors before (level 1)	After
src/wp-admin/plugins.php	40	0
src/wp-admin/users.php	25	0
src/wp-admin/user-edit.php	15	0
src/wp-includes/user.php	18	0

Verification

vendor/bin/phpstan analyse --level=1 --memory-limit=2G \
  --configuration=phpstan.neon.dist \
  src/wp-admin/plugins.php \
  src/wp-admin/users.php \
  src/wp-admin/user-edit.php \
  src/wp-includes/user.php

Expected: [OK] No errors

Trac ticket: https://core.trac.wordpress.org/ticket/64898

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[github-actions]

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