Fix: Ensure popover modals in block editor display above metaboxes

Problem:
When metaboxes are attached to the bottom of the screen in the WordPress block editor, popover modals (e.g., link editing popovers) within the `.editor-visual-editor` element are incorrectly rendered beneath these metaboxes. This issue makes the popovers partially or fully obscured, leading to usability problems, particularly when editing links within blocks.

Solution:
Added a `z-index: 9;` to the `.editor-visual-editor` CSS rule. This change ensures that the `.editor-visual-editor` element and its child popovers are displayed above any metaboxes, preventing them from being obscured and maintaining their usability.

Affected Files:
- Updated the relevant SCSS file(s) to include the `z-index` property.

This commit resolves the issue where popover modals are not fully visible due to being rendered underneath subsequent containers. 
WordPress#64957

packages/editor/src/components/editor-interface/style.scss

@@ -9,4 +9,5 @@
 
 .editor-visual-editor {
 	flex: 1 0 auto;
+	z-index: 9; /* Added z-index to ensure popovers are displayed above other elements such as metaboxes */
 }