\Drupal\Core\Entity\ContentEntityStorageBase::createWithSampleValues() does not handle language or bundles correctly

Problem/Motivation

*I encountered this issue while generating content with devel_generate. I used claude to diagnose, patch, and update the summary.

ContentEntityStorageBase::createWithSampleValues() generates a random sample value for every field
except the ID, revision and bundle keys, including fields the entity
system is supposed to manage itself. This causes several problems:

1. default_langcode gets a random boolean. About half the time a sample entity is
   created with default_langcode = FALSE. Once saved, it has no
   default translation row, so it loads with every field empty: label() returns NULL, and so do getChangedTime()
   and the rest. Tests that save sample entities fail
   at random because of this.
2. Other managed fields are randomized too. langcode is picked from every language,
   including und and zxx, ignoring the bundle's
   configured default. The revision metadata fields and revision_translation_affected get random values.
   changed gets a random past timestamp that can fall before
   created.
3. The bundle is required even when the entity type supplies one. For example, MenuLinkContent::preCreate()
   adds a default bundle, but
   createWithSampleValues() throws when none is passed.
4. generateSampleValue() can return NULL, and that overwrites a field's default value.
   Many field types don't implement it and return NULL from the base
   class, and FieldItemList::generateSampleItems() sets the field to that NULL, replacing its default.

This affects real sites through EntityReferenceItem::generateSampleValue(). When a reference field has
no entity it can point to, it creates one with
createWithSampleValues(). Devel generate triggers this whenever an auto-create entity reference field
targets a bundle that has no content yet. About half the time, the
auto-created entity is saved without a default translation, and then:

• viewing a page that links to it throws
  

  TypeError: Html::escape(): Argument #1 ($text) must be of type
      string, null given

  from LinkGenerator;

• its edit form throws InvalidArgumentException: The timestamp must be numeric. from
  NodeForm.

Steps to reproduce

1. On the Page content type, add an entity reference field that targets Article, with "Create referenced entities if
   they don't already exist" enabled.
2. Make sure no Article nodes exist.
3. Run drush devel-generate:content 1 --bundles=page.
4. Check the auto-created article:
   

   SELECT nid, langcode, default_langcode FROM node_field_data WHERE type =
       'article'

   . If default_langcode is 0, view the
   generated page while logged in, or open the article's edit form. If it is 1, delete the article and repeat; this
   fails about half the time.

Proposed resolution

In createWithSampleValues():

• Set default_langcode to TRUE unless the caller passes a value.
• Don't generate sample values for langcode, revision_translation_affected, the revision
  metadata keys, or fields of type changed, so they keep
  the values the entity system gives them.
• Make the bundle optional. An unknown bundle still throws.

In FieldItemList::generateSampleItems(), keep the existing value when generateSampleValue()
returns NULL.

Implemented in MR17278. CreateSampleEntityTest now saves and reloads every sample entity before validating
it, and new tests cover the default translation and the managed
fields.

Remaining tasks

• Review.
• Decide whether the managed-field changes (langcode, revision metadata, changed) belong
  here or in a follow-up issue.

User interface changes

None.

API changes

createWithSampleValues() no longer throws when no bundle is passed for an entity type that has a bundle
key. It still throws for an unknown bundle.

Data model changes

None.