chore(test): move to @cloudflare/vitest-plugin, the Workers pool's new name

[m0t0r]

Cloudflare renamed @cloudflare/vitest-pool-workers to @cloudflare/vitest-plugin for its 1.0 (workers-sdk#15074). The old name stopped at 0.22.0 on 2026-08-18 and gets no more releases, and Dependabot does not follow a rename, so we had silently stopped updating.

The change is the package name in package.json, the vitest.config.ts import, the tsconfig.worker.json types entry and the D1Migration type in worker/test/env.d.ts.

• 1.3.0 rather than 1.3.1: 1.3.1 and the wrangler it depends on were published today, and pnpm's minimum release age would have added exceptions for both to pnpm-workspace.yaml.
• Still Vitest 4: Vitest 5 support is workers-sdk#15500, open, due as the plugin's 2.0. Vite+ 1.0 pins Vitest 5, so moving to it waits for that. docs/development.md ("Tooling gotchas") says so.
• compatibility_date: the new plugin bundles workerd 2026-09-26 (was 2026-08-15). The Worker tests pass with compatibility_date 2026-09-26, recorded in the docs, but the date stays at 2026-08-20 here because it changes production's runtime.
• Test-only: deploys still use the root wrangler 4.134.0. The old pool's sharp 0.35.2 leaves the lockfile; sharp 0.35.4 stays, through the root wrangler.

Checks

• pnpm typecheck, pnpm lint, pnpm format:check: clean
• pnpm test: 836 passed, 1 skipped; the worker project 190 of 190
• /code-review and /security-review: no findings. Lockfile integrity matches the registry; 1.3.0 has npm provenance from cloudflare/workers-sdk.