[Feature]: Add table caption validation check for core/table block

Which area does this relate to?

Block validation rule

Is your feature request related to a problem?

Currently, the "Core block accessibility rules" module only validates table headers (check_table_headers).

According to WCAG 1.3.1 (Info and Relationships) and Technique H39, data tables should have an associated <caption> to describe their topic or purpose. Without a caption, screen reader users navigating by tables cannot quickly determine what data the table presents before stepping through cells.

Describe the solution you'd like

Add a check_table_caption validation check to core/table in the Core block accessibility rules module:

• Starting Level: warning (advisory, as not every small layout table strictly warrants blocking publication, and administrators can elevate it to error in Validation Settings if desired).
• Behavior:
  • Flags a warning if the caption attribute is empty or contains only whitespace.
  • Automatically clears once a descriptive caption is provided.

Describe alternatives you've considered

Considered making it an error level check, but warning is more appropriate to prevent blocking publishes on simple presentation tables while still encouraging accessibility best practices.

Checklist

• I have searched existing issues to make sure this isn't a duplicate.

Start by locating the Core block accessibility rules module and its existing check_table_headers implementation for core/table. Add the caption validation alongside it, then verify that empty or whitespace-only captions produce a warning and that a descriptive caption clears the warning.