Canvas AI: Template builder agent should be aware of existing components in regions

Overview

The template builder agent currently does not consider the existing components on the page. For example, if the current page contains a hero banner component and the user asks to generate a template for the homepage of an ice cream shop, the agent adds all the components for that template above the hero banner.

Similarly, as observed in https://www.drupal.org/project/canvas/issues/3533079#comment-16290741, when a prompt like “Create a template for the homepage of an ice cream shop website, with a proper header and footer” is used, if content already exists in the header/footer regions, the agent places duplicate components there as well.

This issue is created as a follow-up to discuss how the agent should behave in such cases.

Proposed resolution

User interface changes