[gapic-generator] Standardize and enable linting

[ohmayr]

During the migration of gapic-generator-python to the mono-repo, the lint session was skipped.

Context:
Linting and auto-formatting were not enforced in the original standalone repository. To maintain a "clean" migration (code-neutral move), we have deferred the initial formatting and linting pass to this follow-up issue.

Tasks:

1. Run nox -s format to apply mono-repo style standards to the generator.

2. Resolve any legacy flake8 or ruff violations that existed in the split repo.

3. Remove session.skip() from packages/gapic-generator/noxfile.py.