[Workers] Correct cloudflare:test type path

[ting-hong-shieh]

Summary

The Workers Vitest setup text referenced the package root, which does not export the ambient cloudflare:test declarations. It now uses @cloudflare/vitest-pool-workers/types, matching the tsconfig.json example and current package exports.

Fixes #30069

Validated with pnpm run check, pnpm run format:check, pnpm run build, and pnpm run test.

Documentation checklist

• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

[cloudflare-docs-bot]

Review

✅ No issues found in commit 6ae9a14.

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[ting-hong-shieh]

Rebased onto current production, and the change is now smaller than when this
was opened.

Since then the docs moved from @cloudflare/vitest-pool-workers to
@cloudflare/vitest-plugin, so the original diff no longer applied. The
underlying problem survived that rename: the prose still names the bare package,
and the bare package does not declare cloudflare:test.

From the published manifest for @cloudflare/vitest-plugin@1.1.2:

"exports": {
  ".":       { "types": "./dist/pool/index.d.mts" },
  "./types": { "types": "./types/cloudflare-test.d.ts" }
}

The root export is the pool API; cloudflare:test is declared in ./types.

The page already contradicts itself on this — the example tsconfig.json a few
lines below the sentence uses "@cloudflare/vitest-plugin/types". This makes the
prose match the example, and is now a one-line change.