[vitest-pool-workers] Fix JSON require() in module fallback

[ksawaneh]

Fixes #9822.

@cloudflare/vitest-pool-workers's module fallback service returned .json files as commonJsModule source. When a CommonJS dependency does require("./data.json"), workerd would try to parse the JSON as JavaScript and throw SyntaxError: Unexpected token ':'.

This change:

• Extends fallback resolution to consider .json
• Serves .json as an ES module wrapper that exports JSON.parse(...) (avoids object-literal __proto__ prototype mutation)
• Adds a regression test covering CommonJS JSON require
• Adds a changeset for @cloudflare/vitest-pool-workers

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:

Commands run:

• pnpm check

• pnpm --filter @cloudflare/vitest-pool-workers run check:lint

• pnpm --filter @cloudflare/vitest-pool-workers run check:type

• pnpm exec vitest run test/json-require.test.ts (from packages/vitest-pool-workers)

• Public documentation

  • Cloudflare docs PR(s):
  • Documentation not necessary because: internal module fallback behavior + regression test, no docs change required

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[changeset-bot]

🦋 Changeset detected

Latest commit: 4a3d561

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