[regression] Recursion detected when merging duplicate #attached page_top/page_bottom attachments on node preview

Problem/Motivation

After #3339905: Add page_top and page_bottom to #attached to get rid of NodeThemeHooks::pageTop(), node preview adds the preview form through:

$build['#attached']['page_top']['node_preview']

On a Drupal 11.4.1 site, node preview can fail with:

Error: Recursion detected in array_merge_recursive() (line 165 of core/lib/Drupal/Core/Render/BubbleableMetadata.php).

The failure happens in:

Drupal\Core\Render\BubbleableMetadata::mergeAttachments()

Debugging showed that both attachment arrays being merged contained the same keyed page region attachment:

#attached['page_top']['node_preview']

Because mergeAttachments() currently falls back to array_merge_recursive() for page_top and page_bottom, duplicate render arrays in those keys can trigger PHP recursion detection.

This does not appear to be content recursion. The recursion is in the attachment merge, not repeated entity rendering.

Steps to reproduce

Observed on Drupal core 11.4.1.

1. Have a node type with preview enabled.
2. Use a full view display with nested/rendered content, for example paragraphs/entity reference revisions and Display Suite.
3. Edit a node of that type.
4. Click Preview.
5. The preview route, e.g. /node/preview/{uuid}/full, returns a 500 error.

Observed error:

Error: Recursion detected in array_merge_recursive() (line 165 of core/lib/Drupal/Core/Render/BubbleableMetadata.php).
Drupal\Core\Render\BubbleableMetadata::mergeAttachments()
Drupal\Core\Render\BubbleableMetadata->merge()
Drupal\Core\Render\RenderContext->update()
Drupal\Core\Render\Renderer->doRender()
Drupal\Core\Render\MainContent\HtmlRenderer->prepare()

In the failing request, the duplicate attachment was:

#attached['page_top']['node_preview']

Proposed resolution

Handle page_top and page_bottom in BubbleableMetadata::mergeAttachments() before the generic array_merge_recursive(), similar to the existing special handling for placeholders.

These attachment keys are keyed page-region render arrays and should not be recursively merged when the same key exists on both sides.

Remaining tasks

• Add or confirm a failing test case for duplicate #attached['page_top'] attachments.
• Review whether page_bottom needs identical handling.
• Review whether keyed union is the correct merge behavior for duplicate page region children.

User interface changes

None.

Introduced terminology

None.

API changes

No public API changes intended.

Data model changes

None.

Release notes snippet

Fixes a fatal error on node preview where duplicate #attached['page_top'] attachments could cause recursion during render attachment merging.