Add Dependabot configuration for pip and GitHub Actions (#2041)

* Add Dependabot configuration for pip and GitHub Actions

Enables automated dependency and security (CVE) update PRs for the pip
test requirements and the GitHub Actions used in workflows.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review feedback on dependabot config

- Clarify header comment: this file enables version updates only; CVE
  security updates require the separate repo Settings toggle.
- Drop custom 'python' and 'github-actions' labels that Dependabot would
  silently ignore; keep only the auto-created 'dependencies' label.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>