fix(material-experimental/theming): Fix token causing test failure on CI (#28569)

Author: mmalerba

src/material-experimental/theming/_custom-tokens.scss

@@ -344,7 +344,7 @@
     calendar-period-button-text-color: map.get($systems, md-sys-color, on-surface-variant),
     calendar-period-button-icon-color: map.get($systems, md-sys-color, on-surface-variant),
     calendar-navigation-button-icon-color: map.get($systems, md-sys-color, on-surface-variant),
-    calendar-header-divider-color: transparent,
+    calendar-header-divider-color: _hardcode(transparent, $exclude-hardcoded),
     calendar-header-text-color: map.get($systems, md-sys-color, on-surface-variant),
     calendar-date-today-outline-color: map.get($systems, md-sys-color, primary),
     calendar-date-today-disabled-state-outline-color: mat.private-safe-color-change(