Rewrite Rules: Fix rewrite rules breaking for taxonomies with URL-encoded Unicode slugs

[SainathPoojary]

This PR intercepts the permastruct inside WP_Rewrite::add_permastruct() and decodes the URL-encoded slug back into standard UTF-8 text before saving it.
To ensure we don't accidentally corrupt real rewrite tags (where %ca in %category% could be decoded by a naive urldecode), we use a preg_replace_callback that strictly decodes only uppercase hex strings ([0-9A-F]). WordPress's URL encoding always uses uppercase, while built-in rewrite tags use lowercase, making this a safe and robust fix.

Trac ticket: #41791

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