fix: #3610847 Menu link titles and field labels are double-escaped since 2.0.17

LinksPropType::normalize() and StringPropType::normalize() escape plain strings since the SA-CONTRIB-2026-075 fix, but they can run twice on the same value: MenuSource and FieldLabelSource normalize their output at the trust boundary, and TwigExtension::normalizeProps() normalizes the prop value again at render time. The second pass re-escaped the already escaped string, so a menu link titled 'Aufenthalt & Besuch' rendered as 'Aufenthalt & Besuch'.

Make LinksPropType title escaping idempotent by wrapping the escaped title in Markup::create(), drop the redundant pre-normalization in FieldLabelSource, and stop preprocess() from marking never-normalized plain-string titles as safe.

Closes #3610847