python: should add integration workflow to run generate -all

[zhumin8]

Looks like https://github.com/googleapis/librarian/blob/main/.github/workflows/python.yaml only runs unit test.

We should setup integration test workflow to run librarian generate -all with librarian HEAD on google-cloud-python repo. This can help us catch issues when for example generator version is upgraded in librarian repo.
Possible cause for #6450 (still investing, may not be true. Nevertheless, this test helps with such situations)

Similar to https://github.com/googleapis/librarian/blob/main/.github/workflows/java.yaml or https://github.com/googleapis/librarian/blob/main/.github/workflows/go.yaml