filter page list overlay styles through safecss_filter_attr

[nvxbug]

Repro: add a Navigation block with a Page List inside it and give the Navigation a custom overlay color carrying extra declarations, e.g. set customOverlayBackgroundColor to red;behavior:url(#default#time2);background:url(javascript:alert(1)). The submenu <li> renders that whole string verbatim in its style attribute for every front-end visitor.

Cause: block_core_page_list_render_nested_page_list() emits the overlay style with esc_attr() alone. The overlay colors arrive as plain Navigation context strings, so they never pass through the style engine, get_block_wrapper_attributes(), or KSES, and the CSS property allowlist is skipped.

Fix: wrap the value in safecss_filter_attr() before esc_attr(), matching how the Navigation block renders these same overlay colors.

Trac ticket: none yet; happy to open one to track this.

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