What?

Closes #70611

Adds CSS class support to modal children container when using fill size or fullscreen mode to enable proper height styling.

Why?

Currently, when using the Modal component with size="fill" or isFullScreen={true}, the children container div doesn't have proper height styling. This makes it difficult for developers to create content that takes up the full height of the modal, as the container collapses to its content height rather than expanding to fill the available space.

The issue affects developers who want to create modals with content that needs to utilize the full available height, such as forms with bottom-positioned buttons or scrollable content areas.

How?

• Added conditional CSS class is-fill-size to the modal's children container when size="fill" or isFullScreen={true}
• This class can be targeted by developers to apply min-height: 100% or similar styles to make content fill the available modal height
• Only applying the class when fill sizing is explicitly requested
• Added comprehensive tests to verify the class is applied correctly in all scenarios

Testing Instructions

1. Open the Modal component storybook: https://wordpress.github.io/gutenberg/?path=/story/components-modal--default
2. Set the modal size to "fill" using the storybook controls
3. Verify that the modal's children container now has the is-fill-size CSS class
4. Test with isFullScreen={true} to ensure the class is also applied in fullscreen mode
5. Verify that regular modal sizes (small, medium, large) do not have the is-fill-size class
6. Test the new functionality by adding content with height: 100% to see it now properly fills the modal height

Testing Instructions for Keyboard

1. Navigate to the Modal storybook using Tab key
2. Use arrow keys to change the size control to "fill"
3. Verify the modal opens and closes properly with Escape key
4. Test focus management within the modal using Tab key
5. Ensure the new CSS class doesn't interfere with existing keyboard navigation

Screenshots