chore: cleanup remaining sx props

[francinelucca]

Borrowing changes from #6929 that don't have blockers and can be merged now

Relates to https://github.com/github/primer/issues/5424 and https://github.com/github/primer/issues/5828

Changelog

Removed

• sxProp and BoxWithFallback from components that no longer support it

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

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 8dbc374

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Patch
@primer/styled-react	Patch

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[Copilot]

Pull Request Overview

This PR removes the remaining sx prop support from various components as part of the cleanup effort to migrate away from the legacy styled-components system to CSS modules. The changes eliminate SxProp type annotations, remove BoxWithFallback dependencies, and update Storybook configuration to exclude the removed props.

Key changes:

• Removed sx prop support from internal components and helper utilities
• Replaced BoxWithFallback usage with native HTML elements in affected components
• Updated Storybook controls to exclude the removed sx prop from various component stories

Reviewed Changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/react/src/utils/story-helpers.tsx	Removed 'sx' from textInputExcludedControlKeys array
packages/react/src/utils/form-story-helpers.tsx	Removed 'sx' from textInputExcludedControlKeys array
packages/react/src/internal/components/UnstyledTextInput.tsx	Removed SxProp type and BoxWithFallback, replaced with native input element
packages/react/src/internal/components/CheckboxOrRadioGroup/CheckboxOrRadioGroupValidation.tsx	Removed SxProp type and sx prop usage from component
packages/react/src/internal/components/CheckboxOrRadioGroup/CheckboxOrRadioGroupCaption.tsx	Removed SxProp type and replaced with HTMLSpanElement props
packages/react/src/internal/components/CheckboxOrRadioGroup/CheckboxOrRadioGroup.tsx	Removed SxProp, BoxWithFallback usage, and sx prop; replaced with dynamic component selection
packages/react/src/UnderlineNav/UnderlineNav.stories.tsx	Removed 'sx' from excludedControlKeys array
packages/react/src/RadioGroup/RadioGroup.stories.tsx	Removed 'sx' from Storybook controls exclude list
packages/react/src/Radio/Radio.stories.tsx	Removed 'sx' from excludedControlKeys array
packages/react/src/CheckboxGroup/CheckboxGroup.stories.tsx	Removed 'sx' from Storybook controls exclude list
packages/react/src/AvatarStack/AvatarStack.tsx	Removed BoxWithFallback dependency and replaced with native span element

[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/3993

[primer-integration]

🟢 ci completed with status success.