chore: run the adapter-bun unit tests in Bun

[Nic-Polumeyv]

The specs now run under bun test, with real Bun.semver, Bun.file, and Bun.env, and spyOn recording calls where assertions need them. Bun.serve and the Bun.build pipeline stay test doubles so the tests don't bind sockets or bundle. The build-generated modules (MANIFEST, ROUTES, SERVER, SERVER_OPTIONS) are still mocked, and since mock.module fixes a module's export names on first registration, test/mocks.ts registers each with the union of exports its consumers use. Loads that need a fresh evaluation import with a unique query string, as there is no vi.resetModules.

Bun.version and the Bun global are frozen inside Bun, so the 1.4-floor guard is tested by spying Bun.semver.order plus assertions pinning the real comparator's verdicts, canaries being the reason it uses order(). The build-must-run-in-Bun error comes from running the adapter under a Node subprocess. CI installs Bun 1.4.0 in the test:others legs.

Stacked on #16880.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from dcb160f:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/dcb160f4dd1cb85e724109d55a374a8a26c4af68

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16913

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Preview: https://svelte-dev-git-preview-kit-16913-svelte.vercel.app/