Fix app deploy including stale dist files in the extension bundle

[alfonso-noriega]

WHY are these changes introduced?

Related to #6069.

Since 3.94.0 (0986095), executeBundleUIStep builds the UI extension into the extension's local dist/ directory and then copies the whole directory into the deploy bundle. Any stale file sitting in dist/ (a script built under an old handle, a dev's own bundler output, old sourcemaps/metafiles) ships inside the module.

For web pixel extensions this is fatal: core's FetchScriptContent rejects modules whose uploaded files contain more than one .js, so appVersionCreate fails with a generic "Version couldn't be created" even though every prior step (build, GCS upload) succeeds. dist/ is gitignored, so reverting source changes doesn't clear it — which is why "the same source deployed fine before and now fails".

Root cause analysis in this Slack thread. Server-side error message improvement in https://github.com/shop/world/pull/967972.

WHAT is this pull request doing?

• bundle-ui-step.ts: instead of copying the entire local output directory, copy only the artifacts produced by the build — <output-name>.* for the main entry and each extra asset (which also covers their sourcemaps and .metafile.json). Stale files next to the built ones no longer ship.
• Adds two tests: a stale .js in the local dist/ is excluded from the bundle; extra built assets (e.g. -conditions.js) are still copied.

How to test your changes?

1. Create an app with a web pixel extension and deploy it once.
2. Rename the extension handle in shopify.extension.toml (leaving the old extensions/<pixel>/dist/<old-handle>.js in place) and run shopify app deploy.
3. Without this fix, appVersionCreate fails ("Version couldn't be created"); with it, only the freshly built script is included and the deploy succeeds.

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