[18734] Add canonical redirect for categories with children

[kasparsd]

Trac ticket: https://core.trac.wordpress.org/ticket/18734

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Pull request overview

This PR updates redirect_canonical() so that category URLs for child terms are normalized to use the correct parent/child path, including paginated archives, and adds fixtures and tests to cover these cases (Trac #18734).

Changes:

• Extend Tests_Canonical::data_canonical() with cases for child categories where the URL is missing parent slugs, has extra parent segments, or includes pagination.
• Add shared fixtures that create posts in the parent, child-1, and child-2 categories to ensure category archives have multiple pages.
• Adjust redirect_canonical() to rewrite the category portion of the path based on the actual term hierarchy and the category permalink structure when handling category queries.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/phpunit/tests/canonical.php	Adds canonical redirection test cases for child category URLs with missing/extra parent segments and for paged category archives.
tests/phpunit/includes/testcase-canonical.php	Extends shared fixtures to create enough posts in the parent, child-1, and child-2 categories to support paged archive tests.
src/wp-includes/canonical.php	Updates redirect_canonical() taxonomy handling to normalize the category path portion of the URL to the correct parent/child hierarchy when %category% is used in the category permastruct.

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