[APPS] Support uploading backend functions to app definitions

[sdkennedy2]

Motivation

High code apps need the ability to define backend functions that get bundled and published to the app definition after upload. This enables server-side logic to be co-located with the app's frontend code and automatically deployed as part of the build process.

Changes

Added a complete backend functions pipeline that runs after a successful app upload:

1. Discovery — scans a configurable backendDir (default: backend/) for function modules. Supports both single-file (backend/myFunc.ts) and directory (backend/myFunc/index.ts) patterns.

2. Bundling — each discovered function is bundled with esbuild into a self-contained ESM module. Node built-ins are externalized. If @datadog/action-catalog is installed, its setExecuteActionImplementation export is forced into the bundle.

3. Transformation — bundled code is wrapped in the Action Platform script format with a main($) entry point that registers the $.Actions-based executeAction implementation and invokes the function with template-resolved args.

4. Publishing — transformed functions are sent as ActionQuery objects to the PATCH /api/v2/app-builder/apps/{app_builder_id} endpoint.

Shared constants (NODE_EXTERNALS, ACTION_CATALOG_EXPORT_LINE, SET_EXECUTE_ACTION_SNIPPET) are extracted into backend-shared.ts for reuse by the upcoming dev server middleware.

New configuration option:

{
  apps: {
    backendDir: 'backend', // directory containing backend function modules
  }
}

QA Instructions

1. Configure an app with a backend/ directory containing one or more function modules
2. Run a production build with APPS_UPLOAD_ASSETS=1 and valid API/app keys
3. Verify the app upload succeeds and backend functions are published to the app definition
4. Confirm the function queries appear in the app definition via the App Builder API

Blast Radius

• Only affects apps that have a backend/ directory — no impact on existing apps without backend functions
• The publish step only runs after a successful upload and when valid auth credentials are present
• Published package.json exports fields were updated to use standard import/require/types conditions

Documentation

• How to reliably make frontend production changes
• PR Description Guidelines
• Deploy steps

[sdkennedy2]

• [APPS] Add vite dev server middleware for local backend function testing #285
• [APPS] Support uploading backend functions to app definitions #284 👈 (View in Graphite)
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.