CEXT-6412: add product/custom-grid-columns v2 sample

[obarcelonap]

Description

Adds the admin-ui-sdk/v2/product/custom-grid-columns sample, migrated from the v1 extension point (commerce/backend-ui/1) to App Management (commerce/backend-ui/2).

Related Issue

N/A

Motivation and Context

The v1 Admin UI SDK registration pattern (iframe-based, API Mesh for data) is superseded by the v2 App Management approach. This sample demonstrates the v2 pattern using app.commerce.config.ts and @adobe/aio-commerce-sdk grid column helpers.

Key changes from v1:

• app.commerce.config.ts replaces the v1 registration action and extension-manifest.json
• commerce/backend-ui/2 extension point replaces commerce/backend-ui/1
• Grid column data is served by a runtime action using parseGridRequest/okGridResponse instead of API Mesh
• No web-src UI — v2 has no frontend bundle
• Dependencies trimmed to @adobe/aio-commerce-lib-app + @adobe/aio-commerce-sdk

How Has This Been Tested?

Manually validated file structure and configuration after running aio-commerce-lib-app init.

Screenshots (if appropriate):

N/A

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.