address png encoding bug, path changes

[JoshuaWhite1]

Closes https://github.com/shop/issues-admin-extensibility/issues/2452

WHY are these changes introduced?

When serving static assets for UI extensions during development, the dev server only looked in the build output directory and the extension's root source directory. Extensions that declare a per-extension-point assets folder (via the assets config key) had no way to serve those static files through the dev server, leaving assets like images inaccessible at runtime.

WHAT is this pull request doing?

Adds a third candidate lookup path when resolving extension assets: the per-extension-point assets directory declared in the extension configuration. The resolution order is now:

1. Build output directory (e.g. dist/handle.js)
2. Extension source root (e.g. tools, instructions)
3. Per-extension-point static asset folders declared via the assets config key (e.g. ./assets/logo.png)

This mirrors how the admin spec's static_root serves a whole directory of static files.

Additionally, readFile is called with an explicit options argument ({}) so that it returns a Buffer rather than a string, and test assertions are updated to use .toString() accordingly.

A new test covers the fallback to the extension-point assets folder, verifying that a static asset (e.g. logo.png) is correctly resolved and served with the appropriate Content-Type.

How to test your changes?

1. Create a UI extension with an extension_points entry that includes an assets key pointing to a local directory (e.g. assets: './assets').
2. Place a static file (e.g. logo.png) in that directory.
3. Run shopify app dev and request the asset via the extension dev server URL.
4. Confirm the asset is served correctly with the right Content-Type header.

Alternatively, run the existing test suite:

pnpm test packages/app/src/cli/services/dev/extension/server/middlewares.test.ts

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

[JoshuaWhite1]

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

• address png encoding bug, path changes #7378 👈 (View in Graphite)
• Add assets config key support for UI extension points #7363
• support bundle folder in the bundle ui step config #7340
• main

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