feat: #3589801 Expose HTTP API for managing Page Regions

Exposes list/get/post/patch API endpoints for page regions. Extracted from https://www.drupal.org/project/canvas/issues/3576837.

The list endpoint limits returned regions to the default theme. The post endpoint allows omitting the region's theme and, if omitted, assumes the default one. This allows implementing regions support in the Canvas CLI push/pull without introducing the concept of a Drupal theme to the local code components project codebase.

AI-Generated: Yes (Used Claude Code to plan and iterate over implementation).

Closes #3589801