Validation error on save with Content Moderation and translatable paragraph types

Problem/Motivation

Saving a node fails with "Non-translatable fields can only be changed when updating the current revision" when these three conditions are met:

1. Content Moderation is enabled on the content type
2. Content Translation is enabled on the paragraph type
3. The paragraph type has non-translatable fields

This is the recommended configuration per [#2951436-67]: paragraph types should be translatable, but the ERR field on the node should not.

Content Moderation creates a non-default node revision (isDefaultRevision=FALSE) during form prepare. ERR propagates this flag to all paragraph entities. On save, Content Moderation corrects the node back to isDefaultRevision=TRUE, but that happens after paragraph validation has already fired. So the paragraphs still carry FALSE, and EntityUntranslatableFieldsConstraintValidator rejects any field changes.

I wrote a more detailed analysis with the full execution flow in [#3150084-70].

This has been discussed in the ERR queue for a long time ([#3150084]). The MR there changes storage behavior in ERR, which raised concerns about regressions ([#3150084-66], [#3150084-67]). As far as I can tell, ERR propagating the flag is correct. The timing problem seems to be in ParagraphsWidget, which runs validation while the paragraphs still carry the wrong flag. So I think the fix makes more sense here.

Steps to reproduce

1. Enable Content Moderation on a content type.
2. Enable Content Translation on a paragraph type used by that content type.
3. Make sure the paragraph type has at least one non-translatable field.
4. Create or edit a node and save.

Proposed resolution

ParagraphsWidget::massageFormValues() runs before validation, so it is possible to correct the flag there. The change is in-memory only. The save process recalculates the flag independently via entity_reference_revisions_entity_revision_create(), so storage and caching should not be affected.

  // In ParagraphsWidget::massageFormValues(), after loading the paragraph entity:
  if ($paragraphs_entity instanceof \Drupal\Core\Entity\RevisionableInterface
    && !$paragraphs_entity->isNew()
    && $paragraphs_entity->isTranslatable()
    && !$paragraphs_entity->isDefaultRevision()) {
    $paragraphs_entity->isDefaultRevision(TRUE);
  }
  

This runs at every nesting level since massageFormValues() is called recursively. I tested this with Published and Draft workflows on Drupal 11.2.9 with nested paragraphs (3 levels deep).

Remaining tasks

1. MR with the fix
2. Automated tests
3. Review

User interface changes

None.

API changes

None.

Data model changes

None.