Validate Medium and Small shadow tiers for Modern UI

## Description

The Figma elevation documentation currently includes only Large and XLarge shadows.

In the product, Modern UI also defines Small and Medium shadow tokens:

- Small: `--vscode-shadow-sm`
- Medium: `--vscode-shadow-md`

However, they behave differently:

- Small is explicitly made transparent when Modern UI is enabled, so it has no visible Modern UI use case.
- Medium remains active and appears on surfaces such as the editor minimap edge, Extension item hover, Chat Editing overlays, and inline diff controls.

## Validation needed

Please confirm whether **Medium shadow** is an intentional part of the Modern UI elevation system.

If Medium is valid:

- Define its intended semantic role and usage boundaries.
- I will add Medium shadow to the Figma documentation.

If Medium is not valid:

- Identify whether its product uses should move to Large, use a border, or remove elevation.

Please also confirm that **Small shadow is intentionally unsupported in Modern UI** and does not need to be documented in Figma.

## Acceptance criteria

- A decision is documented on whether Medium is a supported Modern UI shadow tier.
- If approved, Medium’s intended usage is defined for the Figma documentation.
- If rejected, existing Medium product uses are identified for alignment.
- Small is confirmed as either intentionally excluded or requiring further alignment.
- The product and Figma elevation systems use the same supported tiers.

Currently in product