{% trans %} does not support render array and MarkupInterface valued placeholders

Rebased MR against 11.x, and h/t alexpott for changing MR target branch.

Changed to use an inline template test instead. All tests passing now.

Per note from alexpott on Slack, pushed commits to clean up some todos around this issue.

So tested this by kinda following the CR (which updated the version numbers)

Copied
$variables['render_array'] = [
'#prefix' => '',
];

Into olivero_preprocess_page_title and placed

{% trans %}
This is a {{ render_array }}.
{% endtrans %}

In the page_title.html.twig file.

Got a fatal error

Applied the MR

Now the error is gone and "This is a trans render array" appears just fine.

Updating credit

I read the comments, the MR and the CR. I didn't seen any unanswered questions. I don't see a review of the change record and I made an edit anyway so I am tagging for that for review.. There is one question in the MR.

Addressed MR comment. I also removed custom assert messages per 🌱 [META] [policy] Remove PHPUnit assertion messages when possible, and standardize remaining messages Active in the tests.

Also updated the CR and the issue title, because I think the word "placeholder" is confusing. Not sure exactly what Twig's terminology is for {{ ... }} syntax, but from the docs, there's this:

There are two kinds of delimiters: {% ... %} and {{ ... }}. The first one is used to execute statements such as for-loops, the latter outputs the result of an expression.

I've updated language in this issue title to replace "placeholder" with "expression" or "rendered expression" for lack of better terminology.