Inconsistency: page validate passes but prebuild fails with invalid content error

[perasperaactual]

Description

`pages/content.yml` contains a grid section with columns.content_items that failed prebuild with:

Error: Invalid content: /home/charles/git/peraspera/www/pages/content.yml
  content.content_items.4: Invalid input

However, `stackwright page validate` reported "All pages are valid" for the same file.

Steps to reproduce:

1. Create a grid content item with columns where content_items lacks required fields:
   • missing `type` field
   • missing `label` field
   • missing `textBlocks` wrapper for text_block items
2. Run `stackwright page validate pages/content.yml` → passes
3. Run `stackwright prebuild` → fails

Expected behavior:
Both commands should use the same validation logic so they produce consistent results.

Actual behavior:

• `page validate` silently accepts malformed grid column content_items
• `prebuild` correctly rejects them

Workaround used:
Wrap text in proper text_block structure with type, label, and textBlocks array.