[APPS] Add vite dev server middleware for local backend function testing

[sdkennedy2]

Motivation

Developing backend functions requires a fast local feedback loop. Without a dev server, developers would need to upload and deploy to test every change. This middleware enables local execution of backend functions against the Datadog Action Platform API directly from the Vite dev server.

Changes

Added Vite dev server middleware that registers two POST endpoints:

• /__dd/executeAction — bundles a backend function on-the-fly with esbuild, transforms it into the Action Platform script format, executes it via the Datadog single_action_runs API, and polls for the result. Accepts { functionName, args? } in the request body.

• /__dd/debugBundle — returns the raw bundled code for a backend function, useful for inspecting what gets sent to the Action Platform.

The bundling pipeline mirrors the production backend-functions.ts approach (reusing shared constants from backend-shared.ts) but adds dev-specific behavior:

• Reads files at request time for instant feedback on code changes
• Detects default exports and re-exports them as named exports for the script wrapper
• Includes a lodash import preamble matching the Action Platform runtime environment

The middleware is registered via unplugin's vite.configureServer hook and only activates during Vite dev server usage — no impact on production builds or other bundlers.

QA Instructions

1. Set up a high code app with a backend/ directory containing a function module
2. Configure valid DD_API_KEY, DD_APP_KEY, and DD_SITE environment variables
3. Run vite dev with the Datadog plugin configured
4. Send a POST request to http://localhost:<port>/__dd/executeAction with { "functionName": "myFunc", "args": [1, 2] }
5. Verify the function executes and returns a result
6. Test /__dd/debugBundle with { "functionName": "myFunc" } to inspect the bundled output

Blast Radius

• Only active during Vite dev server mode — zero impact on production builds
• Only affects apps using the Datadog vite plugin with valid auth credentials
• Adds vite as a dependency to the apps plugin package

Documentation

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

[sdkennedy2]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

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

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