Fix invalid breadcrumb JSON-LD schema on taxonomy pages

[duncanmcclean]

This pull request fixes an issue where the BreadcrumbList JSON-LD schema rendered an invalid itemListElement on taxonomy term pages — an object with non-sequential numeric keys and gaps (e.g. {"0":{...position 1...},"2":{...position 3...}}) instead of a sequential array.

This was happening because the nav:breadcrumbs tag drops any taxonomy crumb whose template doesn't exist, and Laravel's reject() preserves the collection's keys. When SEO Pro then mapped over the breadcrumbs, it used the collection key as the position, so the leftover gapped keys produced non-sequential positions and ->all() serialised the result as a JSON object rather than an array. Regular entries weren't affected because every URL segment resolves to a viewable page, so nothing gets dropped.

This PR fixes it by calling ->values() on the breadcrumbs before mapping, re-indexing the collection so positions are contiguous (1..n) and the output is always a sequential JSON array.

Fixes #608