fix(tests): add missing package.json to the action-catalog E2E stub

Vite/Rollup needs a package.json to resolve the bare-specifier subpath
import '@datadog/action-catalog/action-execution', but Node's
require.resolve (used by isActionCatalogInstalled) doesn't. The stub
was missing one, so the check reported the package as installed while
the actual build failed to resolve it - previously masked because the
apps plugin's buildRoot was stale and never pointed at this fixture's
node_modules until the prior commit's fix corrected it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Scott-Meyer