chore(test): replace is-my-json-valid with ajv

[appsir2016-lab]

Replaces is-my-json-valid in the tests with ajv + ajv-formats, the validator the library already depends on, via a small helper in test/utils/validator.js. is-my-json-valid is removed from devDependencies.

Switching validators surfaced three places where the old one silently accepted output that ajv does not. None of them changes library behaviour; the tests now pin what the serializer does today:

1. format: 'time' is rendered as HH:mm:ss with no timezone (asTime in lib/serializer.js). RFC 3339 requires a timezone, so ajv-formats rejects it in both full and fast mode. The helper registers a time format that matches the documented shape.
2. { type: ['string', 'null'], const: 'baz' } with input null serializes to null. In JSON Schema const applies regardless of type, so ajv reports this as invalid. test/const.test.js now asserts the mismatch explicitly instead of asserting validity.
3. { nullable: true, const: 'baz' } cannot be compiled by ajv at all ("nullable" cannot be used without "type"), so that test no longer validates the output against the schema.

Items 2 and 3 may be worth a separate discussion about whether the serializer should honour const before type: null. This PR does not change that.

is-my-json-valid did not implement const, which is why these went unnoticed. The old validator also ignored the fast-json-stringify specific format: 'unsafe'; the helper registers it as always valid to avoid ajv's unknown-format warning.

Checks run locally: npm run test (516 unit tests, 100% coverage, TypeScript tests) and npm run lint, all passing on Node 26.

Checklist

• run npm run test && npm run benchmark --if-present (benchmark not run: test-only change)
• tests and/or benchmarks are included
• documentation is changed or added (not needed)
• commit message and code follows the Developer's Certification of Origin and the Code of conduct

Closes #581