CEXT-6409: add order/custom-grid-columns v2 sample

[obarcelonap]

Description

Adds the admin-ui-sdk/v2/order/custom-grid-columns sample demonstrating how to implement order grid custom columns using the Admin UI SDK v2 and app management.

Key changes:

• app.commerce.config.ts — declarative config with adminUi.order.gridColumns wiring order-custom-grid-columns/get-order-grid-columns as the runtime action
• Runtime action updated to use parseGridRequest, okGridResponse, and errorGridResponse from @adobe/aio-commerce-sdk/admin-ui/grid-columns (v2 wire contract)
• Generated app management actions (app-config, association, installation) under commerce/extensibility/1
• All actions on nodejs:22; dependencies trimmed to @adobe/aio-commerce-lib-app and @adobe/aio-commerce-sdk
• v1 source (commerce-backend-ui-1, mesh.json, schema_orders.json, extension-manifest.json) removed

Related Issue

CEXT-6409

Motivation and Context

Provides a clean v2 reference sample for order grid custom columns that uses app management instead of the legacy v1 registration pattern.

How Has This Been Tested?

Deployed to the CEXT6409v2 workspace in the CEXT 6400 stage project and verified extension points commerce/backend-ui/2 and commerce/extensibility/1 are registered successfully.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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.