[Refactor] Simplify copyDirectoryContents implementation

[gonzaloriestra]

WHY are these changes introduced?

copyDirectoryContents in packages/cli-kit/src/public/node/fs.ts reimplemented directory copying by hand: it created the destination, ran a glob over the source tree, and fanned out one copyFile per match through Promise.all. That extra logic is harder to reason about and can hit EMFILE ("too many open files") on large trees because every file is opened concurrently. fs-extra's fsCopy already handles directory creation, recursion, and back-pressure correctly.

WHAT is this pull request doing?

• Replaces the manual glob + Promise.all/copyFile loop in copyDirectoryContents with a single fsCopy(srcDir, destDir) call.
• Keeps the existing source-existence check so callers still get the same error message when the source directory is missing.
• Removes the redundant mkdir for the destination (handled by fsCopy).

How to test your changes?

copyDirectoryContents is exercised by the include_assets build step for UI extensions when the source is a directory. Build any app whose extension declares an include_assets entry pointing at a directory and confirm the contents are copied recursively:

pnpm install
pnpm shopify app build --path <path-to-app>

After the build, verify under the extension's output directory (<path-to-app>/dist/<extension>/...) that:

• the target directory exists,
• all nested files and subdirectories from the source are present (counts and contents match the source),
• the build completes without EMFILE / ENOENT errors.

Negative path — make sure the original error message is preserved when the source is missing. Run from the repo root against the built bundle:

pnpm nx build cli-kit
node -e "import('./packages/cli-kit/dist/public/node/fs.js').then(({copyDirectoryContents}) => copyDirectoryContents('/tmp/does-not-exist-xyz', '/tmp/dest').catch((err) => console.log(err.message)))"
# Expect: Source directory /tmp/does-not-exist-xyz does not exist

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

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