[6.x] Fix custom term templates on collection-scoped term URLs

[duncanmcclean]

This pull request fixes an issue where setting a custom term template on a taxonomy (eg. term_template: taxonomy/term) caused collection-scoped term URLs like /blog/tags/foo to 404, while /tags/foo and /blog/tags continued to work.

This was happening because LocalizedTerm::template() always prefixed the taxonomy's term template with the collection handle when the term was scoped to a collection, producing blog.taxonomy/term. That view never exists, so the view()->exists() check in toResponse() failed. The taxonomy index didn't have this problem since Taxonomy::template() already returns a custom template as-is.

This PR fixes it by only adding the collection prefix when the taxonomy is using its conventional default term template, so a custom term_template is used as-is for both /tags/foo and /blog/tags/foo.

Fixes #10478