fix-action-menu-split-button-click.md

@primer/react-brand	minor

Multiple ActionMenu component updates:

• Calls consumer-provided onClick handlers in default and split-button modes.
• ⚠️ Breaking change to ActionMenu.Button in split-button mode. Custom HTML attributes are now correctly forwarded to the primary action instead of the menu toggle. This was the originally intended behavior.
• Forwards the variant prop in default mode correctly. ActionMenu can now render in all available Button variants.
• Forwards the leadingVisual prop in default mode.