fix(ui5-dynamic-page): add dynamic aria-label based on header state

[NakataCode]

Added dynamic aria-label functionality to the ui5-dynamic-page-header-root that automatically updates based on the header's snap state.

• Added private _snapped property to DynamicPageHeader to track internal header state
• Enhanced _headerRegionAriaLabel getter to return state-specific aria-labels:
  • "Header Expanded" when header is in expanded state (_snapped=false)
  • "Header Snapped" when header is in collapsed state (_snapped=true)
• Updated DynamicPage.onBeforeRendering() to propagate snap state to header component
• Added tests to verify aria-label behavior in both states

Fixes: #12274