test: assert that pages don't embed env when dynamic public env vars are unused

[Nic-Polumeyv]

closes #8946

The follow-up I promised in #16024, a few weeks late. Retargeting it onto version-3 turned it into a test, because the fix I wrote for main is already there. The env embed sits behind the same client.uses_env_dynamic_public check that the $env/dynamic/public branch has used since #11277:

if (client.uses_env_dynamic_public) {
	properties.push(`env: ${load_env_eagerly ? 'null' : devalue.uneval(env.rendered_env)}`);
}

What's left is the regression coverage. On kit 2 with experimental.explicitEnvironmentVariables enabled, every server-rendered page ships an env: {} object the client can never read, since static values are inlined into the generated $app/env/public module as literals. Rich said as much reviewing #15934, "rendered_env is what gets sent from the server to the client. static variables are already in the client".

One assertion added to the existing options-2 prerender test, so version-3 can't regress into shipping the dead object again. Verified it passes on version-3 and that the same assertion fails on main without the omitted fix.

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Install the latest version of @sveltejs/kit from 7850e95:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/7850e95ced127a4eab43a2d15228e3a2a9f5d1c9

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16306

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[teemingc]

Can you please rebase all of your PRs onto the version-3 branch? We have stopped developing against the main branch as we're trying to get SvelteKit 3's release out soon

[Nic-Polumeyv]

Not a problem with some AI help, sorry for this! Thanks for the heads up.

[Nic-Polumeyv]

Heads up on why the diff shrank: version-3 already has this fix. a524898 put the env embed behind client.uses_env_dynamic_public, and #16024 implemented that flag for explicit env. What was left after rebasing is the regression test, retitled accordingly.