Fix: Ensure consistent margin application for blocks with dynamic block appender

[Sourav61]

Ensured consistent margin application for blocks when block list appender appears

What?

Closes #61846

Fixes an issue where the .block-list-appender dynamically modifies the block structure, causing inconsistent margin application for blocks inside .is-layout-constrained containers. This ensures that margins are correctly applied even when the block appender is added or removed dynamically.

Why?

Previously, when a block (e.g., Cover block) was inside a Group block with .is-layout-constrained, the following CSS rules were applied:

:root :where(.is-layout-constrained) > :first-child {
    margin-block-start: 0;
}

:root :where(.is-layout-constrained) > :last-child {
    margin-block-end: 0;
}

However, when selecting the Group block in the editor, the .block-list-appender gets appended to the group, making it the new :last-child, which removes the intended margin adjustments from the actual content block, causing a layout shift.

How?

• Used :has(> .block-list-appender) to detect when a .block-list-appender is present inside .is-layout-constrained.
• Applied :nth-last-child(2) to target the second-to-last element, ensuring the last actual content block (not the appender) retains the correct margin styles.
• This prevents the .block-list-appender from disrupting the layout when dynamically added or removed

Testing Instructions

1. Add a Cover block inside a Group block in the editor.
2. Apply margin styles via theme.json or global styles.
3. Observe the margins before and after selecting the Group block.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: Sourav61 <sourav08@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.