test: handle null-prototype json fields

[tunnckoCore]

Summary

• update the JSON integration test for null-prototype field objects
• assert the hardened object shape explicitly

Why this was breaking

PR #1066 changed parsed fields/files objects to use Object.create(null) to avoid prototype clobbering.

test/integration/json.test.js was still comparing the returned fields value against a plain object literal with assert.deepStrictEqual(). That assertion checks prototypes too, so the test failed even though the printed contents looked identical.

The thrown assertion then prevented the test from reaching res.end(), server.close(), and done(), which is why Jest also reported a timeout.

Fix

• assert Object.getPrototypeOf(fields) === null
• compare { ...fields } to the expected plain object contents