Allow a markdown tooltip for file decorations

https://github.com/microsoft/vscode/issues/256594#issuecomment-3105061118

From @bamurtaugh:

I'd love if the same yellow dot was included in the tooltip next to Review required at the bottom:

<img width="1592" height="366" alt="Image" src="https://github.com/user-attachments/assets/cb8630c2-ad40-43c7-b8d7-601b1724cb25" />