Ajax: Improve sanitization and screen state in dashboard widget updates.

[liaisontw]

Description
This PR improves the security and reliability of dashboard widget updates via AJAX.

Input Sanitization: Adds sanitize_key() to both pagenow and widget parameters in wp_ajax_dashboard_widgets(). This ensures that only valid keys are processed, preventing potential path traversal or unexpected input issues.

Consistent Global State: Explicitly calls set_current_screen( $_GET['pagenow'] ) before processing the widget logic. This is crucial because, in an AJAX context, the global screen state might be uninitialized or polluted by other global variables (like $post), which can cause certain dashboard widgets (especially those relying on RSS feeds) to fail or behave inconsistently.

These changes ensure that the AJAX request environment closely mirrors a standard dashboard page load, improving compatibility with various WordPress environments and preventing common failures in PHPUnit testing environments where global state pollution is frequent.

Trac ticket: https://core.trac.wordpress.org/ticket/65054

Use of AI Tools
AI assistance: Yes
Tool(s): Gemini
Model(s): Gemini 3 Flash
Used for: Assisted in debugging the PHPUnit test case, specifically regarding output buffering (ob_start) issues and WPAjaxDieException handling within the WP_Ajax_UnitTestCase framework. The final implementation and test logic were reviewed and manually adjusted by me to fit WordPress Core standards.

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[github-actions]

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[apermo]

lgtm