Prevent Avatar from shrinking in Button visuals

[wanxiankai]

Closes #7974

Changelog

New

• Added a patch changeset for the Avatar sizing fix.

Changed

• Set Avatar min-width from the same size variables as width so avatars retain their intended intrinsic width in min-content slots, including Button leading visuals.
• Added a CSS rule test that verifies the non-shrinking min-width declaration is present.

Removed

• None.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

• npm run test -- packages/react/src/Avatar/Avatar.test.tsx --run
• npm exec eslint -- packages/react/src/Avatar/Avatar.test.tsx
• npm exec stylelint -- packages/react/src/Avatar/Avatar.module.css
• npm run test:type-check
• npm exec prettier -- --check packages/react/src/Avatar/Avatar.module.css packages/react/src/Avatar/Avatar.test.tsx .changeset/green-avatars-stand.md

[changeset-bot]

🦋 Changeset detected

Latest commit: a23832e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[liuliu-dev]

This change is causing VRT failures in the AvatarStack custom-size responsive story. https://primer-874780de13-13348165.drafts.github.io/storybook/?path=/story/components-avatarstack-features--custom-size-on-children-responsive

Could we address this regression?