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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[irozum]

Solid fix for a real 9-year-old bug: taxonomies/post types registered with a percent-encoded Unicode slug (WooCommerce does this for cyrillic attribute names) end up with literal %D0%A1... bytes in the permastruct, which generate_rewrite_rules() misreads as tag delimiters, shifting every $matches[n] capture. Decoding in add_permastruct() is the right choke point since it's the single entry for taxonomies, post types, and any plugin's own call, matching the reporter's original diagnosis. It's also a nice improvement over the old 2016 patch on the ticket, which called plain urldecode() — that would convert literal + in a slug into a space; rawurldecode() avoids that, and scoping the regex to uppercase hex avoids colliding with any lowercase built-in tag like %category%.

Checked out the branch and ran the full rewrite group (1388 tests, no regressions) plus lint/PHPStan on the two changed files, all clean. The new test correctly asserts both the decoded struct and that the generated rule maps to $matches[1] rather than a shifted index — the exact symptom from the ticket's screenshots.

Non-blocking: a plugin-registered tag whose name happens to be 2+ uppercase hex letters (e.g. a hypothetical %AB%) would now get silently decoded instead of matched — no core tag is shaped that way, but requiring the decoded bytes to be valid UTF-8 before substituting would close that edge case if anyone wants extra safety.