renovate.sh ignores real failures in `generate-libraries.sh`

We use `renovate.sh` to update the googleapis SHA. It automatically updates the generated code too:

https://github.com/googleapis/google-cloud-cpp/blob/682e0657e270533675163dca54a722526aaae145/external/googleapis/renovate.sh#L72-L75

The `|| true` is needed because `generate-libraries.sh` is intended as a CI script. It is supposed to fail if the code does not match what would happen if we generated the code, and in this case we always generate new code.

It seems we need a way to tell `generate-libraries.sh` to expect code diffs. Or maybe we should refactor the code such that both `renovate.sh` and `generate-libraries.sh` call some shell functions.

Start by reading renovate.sh around lines 72-75 and then inspect generate-libraries.sh to understand how its CI failure status is produced. Reproduce the generation flow and ensure expected generated-code differences do not hide unrelated failures, while the CI validation behavior remains intact.

Written by the indexing model from the issue text.