Add ActionBar.Button for text button overflow support

[Copilot]

ActionBar only supported IconButton children; text buttons (now common in GH design) had no first-class API and no overflow handling. This adds ActionBar.Button, used like ActionBar.IconButton, that collapses into the overflow menu when space runs out.

<ActionBar aria-label="Toolbar">
  <ActionBar.Button>Save</ActionBar.Button>
  <ActionBar.Button leadingVisual={SearchIcon}>Search</ActionBar.Button>
  <ActionBar.Divider />
  <ActionBar.Button disabled>Cancel</ActionBar.Button>
</ActionBar>

Changelog

New

• ActionBar.Button — a text Button subcomponent (variant="invisible") that registers with the existing overflow mechanism. Uses children as the overflow menu label and forwards a component leadingVisual as the menu item's leading icon.
• Exported ActionBarButtonProps.
• Storybook example WithTextButtons; docs metadata; unit tests.

Changed

• Overflow action items now render a leading visual only when an icon is present (text buttons may have none); item key switched from label to the registry id. Internally, the action ChildProps icon is now optional and label accepts ReactNode.

Removed

• N/A

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

Additive, backwards-compatible API.

Testing & Reviewing

Storybook → Experimental/Components/ActionBar/Examples → WithTextButtons. Resize the container to verify buttons overflow into the menu, the label (and optional leading icon) render in the menu, and disabled is respected. Unit coverage added in ActionBar.test.tsx (render, click, disabled, keyboard).

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-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 3f9f4ed

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	Minor

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[github-actions]

⚠️ Action required

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