[SainathPoojary]

Description

When requesting a single theme through the themes REST endpoint (/wp/v2/themes/<stylesheet>), providing a stylesheet query parameter or encoded route path with directory traversal sequences (..) could allow wp_get_theme() to read theme headers from a style.css file outside the wp-content/themes directory.

This PR updates WP_REST_Themes_Controller::_sanitize_stylesheet_callback() to normalize path separators with wp_normalize_path() and check for .. segments after URL decoding. If directory traversal is detected, a WP_Error (rest_invalid_stylesheet) is returned, causing the REST API parameter validation to reject the request with HTTP 400 (rest_invalid_param) before theme resolution or permission checks run.

Unit tests have been added to verify that traversal variations are rejected while legitimate stylesheet identifiers containing dots or special characters continue to work.

Trac ticket: https://core.trac.wordpress.org/ticket/66167

Use of AI Tools

AI assistance: Yes
Tool(s): Github Copilot
Model(s): Claude Sonnet 4.6
Used for: Initial code suggestions and tests, final implementation and tests were verified and edited by me.

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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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