[Run] Fix drag

[davidegiacometti]

Summary of the Pull Request

Fix a small WPF .NET9 regression and restore drag of Run window.

PR Checklist

• Closes: [PowerToys Run] Dialogue box only draggable by magnifying glass #36415
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Set a transparent background to the border in order to make the grid firing the MouseDown event.

Validation Steps Performed

Manual test

[jaimecbernardo]

Tested on Windows 11 and Windows 10 and it's working well. Thank you!