Add ActionMenu.IconButton and ActionMenu.Anchor child validation

[Copilot]

ActionMenu.Anchor accepts arbitrary children, making it easy to wrap non-interactive elements (e.g., <div>) and produce inaccessible markup. The most common use case for ActionMenu.Anchor is wrapping IconButton, which should have a blessed shortcut.

This PR adds two things:

1. ActionMenu.IconButton — syntactic sugar (like ActionMenu.Button) that correctly wires an IconButton as the menu anchor
2. ActionMenu.Anchor child validation — dev-only warning when the anchor child is not an interactive element, following the same pattern used in Tooltip

// Before: manual wiring required
<ActionMenu>
  <ActionMenu.Anchor>
    <IconButton icon={KebabHorizontalIcon} aria-label="More actions" />
  </ActionMenu.Anchor>
  <ActionMenu.Overlay>...</ActionMenu.Overlay>
</ActionMenu>

// After: blessed shortcut
<ActionMenu>
  <ActionMenu.IconButton icon={KebabHorizontalIcon} aria-label="More actions" />
  <ActionMenu.Overlay>...</ActionMenu.Overlay>
</ActionMenu>

The validation warns on non-interactive anchors but skips submenu contexts where ActionList.Item is the expected anchor.

Changelog

New

• ActionMenu.IconButton — shortcut for using IconButton as menu trigger, equivalent to wrapping it in ActionMenu.Anchor
• ActionMenuIconButtonProps type export
• Dev-only console.warn when ActionMenu.Anchor receives a non-interactive child element

Changed

• ActionMenu.Anchor now uses useMergedRefs to combine an internal ref (for child validation) with the forwarded ref

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

• New tests cover ActionMenu.IconButton (open/close, event handlers, id forwarding)
• New tests verify the warning fires for non-interactive anchors (<div>) and does not fire for Button or IconButton
• Submenu tests confirm no false-positive warnings for ActionList.Item anchors
• Type tests validate ActionMenu.IconButton prop types

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)

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