Fix modern Bun lockfile cleanup in app templates

[afurm]

WHY are these changes introduced?

No linked issue.

Shopify CLI already detects Bun projects from both bun.lockb and modern bun.lock, but app template cleanup only treated bun.lockb as Bun's lockfile. This could leave stale bun.lock files or .gitignore entries behind when initializing non-Bun app projects.

WHAT is this pull request doing?

• Adds modern bun.lock to the shared lockfile list.
• Adds lockfilesForPackageManager() so Bun keeps both valid Bun lockfile names.
• Updates app template cleanup to remove unused modern Bun lockfiles for non-Bun projects.
• Adds focused regression coverage and a patch changeset.

How to test your changes?

pnpm test packages/app/src/cli/services/init/template/cleanup.test.ts packages/cli-kit/src/public/node/node-package-manager.test.ts
pnpm --filter @shopify/cli-kit type-check
pnpm --filter @shopify/app type-check
pnpm --filter @shopify/cli-kit lint
pnpm --filter @shopify/app lint

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