🐛 [BUG] - Rivers with picture tag do not always have rounded borders

[seangolob]

Describe the bug

Hi there! I am experiencing an issue with the "River" element when I try to leverage the picture tag. The picture element is missing a height: 100% style so it doesn't consistently fill the height of its container. I believe we will need to adjust https://github.com/primer/brand/blob/main/packages/react/src/river/river-shared.module.css#L17 to add support for making the picture tag height: 100%.

I have a workaround in place so this doesn't block me at all.

Reproduction steps

1. Go to https://github.com/social-impact/focus-areas/environmental-sustainability with a logged in GH account
2. Look at the first river element, remove the inline `width` and `height` styles from the `picture` tag.
3. Observe behavior

Expected behavior

I would expect the picture to take up 100 height and not have cutoff edges at the bottom.

Screenshots

Browsers

No response

OS

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