librarian: do not run smoke test in push event

might missed this earlier on other PRs: we are running an extra smoke test on push to main now along side of it test. This seems wasteful as it is already covered by the full test?

_Originally posted by @zhumin8 in https://github.com/googleapis/librarian/pull/7564#discussion_r3983181866_

Inspect the GitHub Actions workflow that handles pushes to main and identify the separate smoke-test and full-test jobs. Confirm how the full test already covers the smoke test, then verify that push events run only the intended test path and that the workflow configuration remains valid.

Written by the indexing model from the issue text.