PR pipeline in azure-rest-api-specs should flag PR with Cadl but not package-lock.json

We should require the package-lock.json file to be present for any Cadl definition. It can either be included in the PR or already present in the repo (not changed), but it should not be absent. We need package-lock.json to ensure that the REST API can be reliably recreated at any point in the future.

Start by locating the PR pipeline used for azure-rest-api-specs and inspect how Cadl definitions are detected. Verify the check distinguishes an included package-lock.json from one already present in the repository, and confirm that a Cadl definition without either is flagged while one with either is accepted.

Written by the indexing model from the issue text.