Plugin block cards concatenate all field values into an unreadable preview

Description

Plugin-defined Portable Text block cards concatenate every string field into one subtitle. Blocks with multiple or long fields become unreadable, and the subtitle truncates to a tiny fragment when the block is selected because the action buttons use the remaining width.

Expected: show a concise, predictable summary, such as a plugin-configurable summary field or one designated display field.

Steps to reproduce

1. Define a native Portable Text block with multiple text inputs, including a multiline field.
2. Insert it and enter long values.
3. View the block in a narrow editor.
4. Select the block.

The subtitle combines every value and truncates to only a few characters.

Environment

• EmDash 0.37.0
• Local Astro dev server
• macOS

Screenshot

At a narrow editor width, the selected card subtitle is reduced to a fragment such as "Copy pr...". The saved content and public rendering are unaffected.