Add Dependabot update cadence

[im-sham]

Summary

• Add Dependabot cadence for GitHub Actions and Python dependencies
• Add regression coverage so the cadence config stays present

Validation

• .venv/bin/ruff check tests/test_ci_workflow.py
• .venv/bin/pytest tests/test_ci_workflow.py -q
• ruby -e 'require "yaml"; YAML.load_file(".github/dependabot.yml")'
• git diff --check -- .github/dependabot.yml tests/test_ci_workflow.py