Metatags are only generated for nodes; other entity routes return empty metatags

Problem/Motivation

CustomElementsMetatagsGenerator::getMetatags() only looks at the node route parameter. For any other content entity route, e.g. taxonomy terms, Canvas pages or other entity types with Metatag defaults, the custom elements response carries empty metatags. The HTML output of the same route does have title, description, canonical link and Open Graph tags, because Metatag supports every content entity type. A decoupled frontend therefore loses all metatags on non-node pages.

Steps to reproduce

1. Enable Metatag and lupus_ce_renderer, with Metatag defaults for taxonomy terms (e.g. the global defaults).
2. Create a taxonomy term.
3. Request /taxonomy/term/{id}?_format=custom_elements.
4. The response has metatags.meta without the term's title and metatags.link without a canonical link. The same request for a node returns both.

Proposed resolution

Fix and add tests

Remaining tasks

• Review.

User interface changes

None.

API changes

New protected method CustomElementsMetatagsGenerator::getRouteEntity().

AI disclosure

This issue and the patch were drafted with AI assistance (Claude Code). A maintainer reviewed and tested the change before posting.