Use @ symbol prefix for responsive style states

[talldan]

Trac ticket: https://core.trac.wordpress.org/ticket/65503
Backport for Gutenberg PR: WordPress/gutenberg#79210

Updates the property keys for responsive style states to include an @ prefix:

• tablet becomes @tablet
• mobile becomes @mobile

This is to avoid the future possibility of property name clashes.

No backwards compatibility is required in WordPress core since responsive style states have not been shipped in any WordPress version. This is a pure rename.

Use of AI Tools

AI assistance: Yes
Tool(s): Codex / OpenCode
Model(s): GPT-5.5
Used for: All code changes, reviewed by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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[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.

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Props talldanwp, isabel_brison.

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[tellthemachines]

LGTM in comparison with the Gutenberg changeset!