Configure Dependabot PRs to skip NuGet publish

[Copilot]

Closes #<ISSUE_NUMBER>

Add Dependabot labels so dependency update PRs automatically include skip-nuget-publish, matching the existing workflow logic that skips publishing when this label is present. This removes the need to manually add the label before merge.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

Validation was run during implementation. Repository build/test currently fails due to pre-existing NU1902 warnings-as-errors on restore, unrelated to this workflow config change.