fix: dispose Cloudflare platform proxy when Vite servers close

[svelte-triage-bot]

Closes #16919

The Cloudflare adapter no longer creates its platform proxy during builds, but the proxy created for Vite dev and preview servers was still retained after those servers closed.

This change retains the proxy in the adapter plugin and disposes it:

• through Vite's awaited closeBundle lifecycle for dev servers
• through the preview server's close() lifecycle
• idempotently when multiple close hooks run
• while clearing the global proxy so a later server can create a fresh instance

Added focused tests covering dev and preview teardown and a patch changeset.

Verification:

• pnpm -F @sveltejs/adapter-cloudflare test:unit
• pnpm -F @sveltejs/adapter-cloudflare check
• pnpm -F @sveltejs/kit test:unit
• pnpm check
• pnpm lint
• Cloudflare workers fixture build, with zero workerd processes before and after

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 2ad1f7e:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/2ad1f7ed00e4a6b017749952a5c06a20f7ba10cd

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16976

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 2ad1f7e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare	Patch

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checking if #16754 already fixed this

[vercel]

Cloudflare adapter unit tests read the vite plugin via plugins?.pre?.[0], but the adapter now returns plugins as a plain array, so the plugin resolves to undefined and the tests throw a TypeError.

[teemingc]

already fixed by #16754