fix: omit env from server-rendered pages when the client doesn't use dynamic public env vars

[Nic-Polumeyv]

closes #8946

The follow-up I promised in #16024, a few weeks late.

With experimental.explicitEnvironmentVariables enabled, every server-rendered page ships this, even when the client can never read it:

<script>
	__sveltekit_y93mhh = {
		base: new URL("..", location).pathname.slice(0, -1),
		assets: "/basepath",
		env: {}
	};

Static values are already inlined into the generated $app/env/public module as literals, and only dynamic ones read the embedded object. 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". The embed just never got the gate.

It now sits behind the same client.uses_env_dynamic_public check the $env/dynamic/public branch has used since #11277. #16024 broadened that flag to cover explicit env vars, which is why this had to wait for it. Dev hardcodes the flag to true, so dev is unaffected, and the dynamic case keeps its env: null deferral while prerendering.

One assertion added to the existing options-2 prerender test. Fails on main, passes with this change, in both halves of the DYNAMIC_PUBLIC_ENV matrix.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from cc99e2d:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/cc99e2d55c9f014c2e73c925013f0b41d32adf7c

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

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