Make Vitest `stubEnv()` work with SvelkeKit `$env/dynamic/public|private`

Describe the problem

As developer I want to stub dynamic env-variables in my Vitest tests, so I can test the app with different env-settings.

https://vitest.dev/api/vi.html#vi-stubenv

Example

.env

FOO=bar

src/lib/config.ts

import { env } from '$env/dynamic/private';

export function getConfig() {
	return env.FOO;
}

src/lib/config.test.ts

import { vi, describe, it, expect } from 'vitest';
import { getConfig } from './config';

describe('getConfig test', () => {
	it('it gets config', () => {
		vi.stubEnv('FOO', 'stub');
		
		console.debug(process.env.FOO);
		const config = getConfig();

		expect(config).toBe('stub');

		vi.unstubAllEnvs();
	});
}

This example is currently not working.

Describe the proposed solution

Currently env-values are copied when the server starts:

https://github.com/sveltejs/kit/blob/9f0617cab5fcdbe2df33c7103ad873d4d318bfc7/packages/kit/src/runtime/server/index.js#L31-L32

Because of this, changing process.env-variables after the server has started will not change the env-values used in the SvelteKit app.

Instead of copying env-values, they could be retrieved via proxy (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) directly from process.env.

The proxy can then decide if the env-variable should be available (public vs. private).

Alternatives considered

Maybe it is possible to stub env-variables before the server starts.

Importance

would make my life easier

Additional Information

No response

Start by reading packages/kit/src/runtime/server/index.js around the linked environment-value copying at lines 31–32, then trace how $env/dynamic/public and $env/dynamic/private expose those values. Done means the provided Vitest stubEnv example observes the stubbed value while public and private availability rules remain correct.