fix(ui5-shellbar): fix full-width search overlay overflow - #14288

Summary

• Replaces bottom: 0.0625rem; left: 0; height: 100%; width: 100% on .ui5-shellbar-search-full-width-wrapper with inset: 0 0 0.0625rem 0
• The previous combination of bottom: 0.0625rem + height: 100% caused the overlay to overflow its container by 1px at the top
• Using inset derives the element's size from its edges, eliminating the mismatch; the 1px bottom gap is preserved so the ShellBar bottom border remains visible

Test plan

• Open ShellBar with search field and expand it — overlay should align flush with the ShellBar on all sides with no 1px overflow at the top
• Verify the ShellBar bottom border is still visible (1px gap at bottom is intentional)

Internal reference: CS20260013772978