Try generating block element selectors only when element styles exist

[tellthemachines]

What?

The description of #76556 states that "For one site I was using to test with I found that there were over 4,000 calls to prepend_to_selector() just to bootstrap WordPress". That sounds like a lot for a function that's only called inside get_block_element_selectors so I looked into it. Turns out we're generating element selectors for every single block that's registered (not even only the blocks on the page that loads), whether it has element styles or not. We shouldn't need to do that.

This PR introduces a check for the existence of element styles, or nested element styles inside breakpoints or style variations for each block, so that we can generate the selectors only for the blocks that need them.

This is not intended as a replacement for #76556, but as a further optimisation.

Testing Instructions

Check that element styles are still being applied correctly to blocks, their viewport state styles and their style variations.

Use of AI Tools

Used codex/gpt 5.5 to validate my initial assessment and make changes based on it. Human reviewed by myself.

[github-actions]

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Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org>

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[github-actions]

Flaky tests detected in a859f86.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/28077257124
📝 Reported issues:

• [Flaky Test] should allow user to add and remove multiple local font files #78760 in /test/e2e/specs/admin/font-library.spec.js