chore(worker): upgrade @cloudflare/vitest-pool-workers to v0.16 and vitest to v4 (#284)

* chore(worker): upgrade @cloudflare/vitest-pool-workers to v0.16 + vitest to v4

Migrates the Cloudflare Worker test setup from vitest 3.x / pool-workers
0.8.x to vitest 4.x / pool-workers 0.16.x:

- package.json: bump @cloudflare/vitest-pool-workers ^0.8→^0.16, vitest
  ^3→^4.1; add "type":"module" (required for vitest 4/rolldown to load
  the ESM config file)
- vitest.config.js: replace defineWorkersConfig({test:{poolOptions:{…}}})
  with defineConfig({plugins:[cloudflareTest(…)]}) per the v3→v4 codemod
- proxy.test.js / beverage-types.test.js: replace fetchMock (removed from
  cloudflare:test in 0.16) with vi.stubGlobal('fetch', vi.fn()) and
  mockResolvedValueOnce/mockRejectedValueOnce; all 56 tests pass

Part of #242 (slices 2 and 3).

* test(worker): assert upstream URL in every proxied fetch call

Each stubbed-fetch test now verifies the exact upstream URL the worker
constructs, restoring the URL-validation coverage that fetchMock's
intercept({ path }) provided implicitly.

* test(worker): assert full upstream fetch args in worker tests

Agent-Logs-Url: https://github.com/richardthe3rd/cambridge-beer-festival-app/sessions/0c00ccf3-adc6-477f-818c-0327541020c6

Co-authored-by: richardthe3rd <573334+richardthe3rd@users.noreply.github.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: richardthe3rd <573334+richardthe3rd@users.noreply.github.com>

Author: 3 people