'breadcrumb' metatags do not point the frontend

Problem/Motivation

breadcrumbs in CE API output have a URL pointing to the backend.

See screenshot, or test-fail MR whose phpunit test outputs

There was 1 failure:

1) Drupal\Tests\lupus_decoupled\Functional\LupusDecoupledApiResponseTest::testSchemaBreadcrumbs
Breadcrumb 0 must start with the frontend base URL.
Failed asserting that 'https://lupus-decoupled.ddev.site:8443/' starts with "https://lupus-nuxt.ddev.site:8443".

Steps to reproduce

• Enable lupus_decoupled_schema_metatag module (or only schema_metatag module; the only difference is in where exactly the breadcrumbs are visible.)
• In configuration - metatag - any node type: Edit settings "Schema.org: WebPage - breadcrumbs", set to Yes
• View API output of corresponding node

Proposed resolution

Discussed:

LupusFrontendPathProcessor::processOutbound() currently only changes URLs that have a (configured) front end path, if these have a 'entity' option.

Instead: always change URLs that have a (configured) front end path. If they don't have a 'entity' option, then change them to the 'default' front end base URL.

Complications with this resolution

Dropping the restriction on $options['entity'] has the effect that several other kinds of links are having the frontend URL appended. Often this is unnecessary. To fix this, $request->getRequestFormat() != 'custom_elements') was basically changed to $request->getRequestFormat() != 'custom_elements') && !$request->attributes->get('lupus_ce_renderer', FALSE) while reviewing this issue.

NOTE - behavior change

The lupus_decoupled_ce_api.frontend_paths services parameter has "/" added. This is necessary for making the breadcrumb links to 'home' in the ld+json payload point to the frontend (not the backend -- they are always absolute).

This has an added effect of making "Home" links on the backend also point to the frontend homepage. They pointed to the backend before.

We basically cannot distinguish one from the other, without adding extra logic.

Complications with this resolution - obsolete comments:

1)The original restriction was;

if ($absolute || $request->getRequestFormat() != 'custom_elements') {
  change to an absolute link pointing to frontend

2)This evolved to;

if ($absolute || !in_array($request->getRequestFormat(), [NULL, 'json', 'custom_elements'], TRUE)) {
  change to an absolute link pointing to frontend

... it's likely fairly obvious that the 'json' format should be exempted from this. But the NULL format is also necessary if we want to prevent redirects specifically implemented through the Redirect module in API output, to be changed from relative to absolute unnecessarily. (Because the redirect module intercepts the REQUEST event, before the lupus_ce_renderer module sets the request format.)

(I'm trying to write a phpunit test to include here, to enforce that links handled by the Redirect module will stay relative. However I'm having some issues with that at the moment.)

3)I am experimenting with instead doing

if ($absolute || !$request->attributes->get('lupus_ce_renderer', FALSE)) {
  change to an absolute link pointing to frontend

Because frankly, it looks more straightforward to me... and it also solves another issue in our internal product.

Some of our internal tests are still failing, but it looks like that might mean our internal code should change instead. To be continued.