Image caption isn't associated to the image in an accessible way #2055

Is your feature request related to a problem? Please describe.
When adding a caption to an image in BlockNote, the caption currently fails to meet WCAG criterion
The issue is that the caption is not semantically associated with its image.

Expected behavior
The <figure> element should wrap both the <img> and its caption (<figcaption>), ensuring a proper semantic relationship.

Describe the solution you'd like

• Use <figure> and <figcaption> for images with captions.
• If a caption is present:
  • Apply the caption as the alt text for the image.
  • Keep the <figcaption> as visible content.
• If no caption is present:
  • Use an empty alt="".
  • Mark the image as decorative with role="presentation" and aria-hidden="true".

Describe alternatives you've considered

• Keeping the <p> caption: rejected, as it does not satisfy WCAG requirements.
• Using aria-describedby: possible fallback, but less semantic than the <figure>/<figcaption> solution.

Additional context
Actual structure :

Example of expected structure:

<figure class="bn-file-block-content-wrapper" role="img" aria-label="Image: Example caption">
  <img src="..." alt="Example caption" />
  <figcaption class="bn-file-caption">Example caption</figcaption>
</figure>