feat(helper-text)!: port to <pf-v6-helper-text> - #3155

Summary

• Removes <pf-v5-helper-text>
• Ports <pf-v5-helper-text> to <pf-v6-helper-text> web component for PatternFly v6
• Displays contextual feedback or validation messages for form fields
• Supports variant (default/indeterminate/warning/success/error), has-icon, dynamic, and screen-reader-text attributes
• Built-in SVG icons from @patternfly/react-icons (rh-ui-minus, rh-ui-warning-fill, rh-ui-check-circle-fill, rh-ui-error-fill)
• Dark mode support via light-dark() CSS fallbacks
• Demos: basic (index), with-custom-icons, multiple, dynamic

Closes #3012

Intentional divergences from React

• Single element instead of React's HelperText (container) + HelperTextItem (item) pair; use plain HTML wrappers with aria-live for grouping
• has-icon attribute for opt-in icon display (React auto-shows icons for non-default variants)
• icon slot replaces React's icon prop (ReactNode → slotted markup)
• screen-reader-text attribute maps to React's screenReaderText prop
• dynamic attribute exposes React's internal .pf-m-dynamic CSS modifier class
• Container props (component, isLiveRegion, aria-label) not implemented; users apply these directly on wrapper elements
• No dependency on <pf-v5-icon> — uses inline SVG icons

Test plan

• npm run test passes
• Demos render correctly at localhost:8000
• All variant styles (colors, font weight, icons) visually match patternfly.org
• Dark mode colors correct when switching color scheme
• Screen reader text announces variant status for non-default variants
• Custom screen-reader-text overrides default announcement
• Empty screen-reader-text="" suppresses announcement
• Custom icon slot displays slotted SVG in place of default icon
• Dynamic demo shows aria-live container with mixed success/error items

Assisted-By: Claude