fix: serialize array input when object precedes array in multi-type schema

[spokodev]

When a schema lists object before array in its type array, array input serializes to {} and every element is lost.

Steps to reproduce

const build = require('fast-json-stringify')

const stringify = build({
  type: ['object', 'array'],
  properties: { a: { type: 'integer' } },
  items: { type: 'integer' }
}, { ajv: { allowUnionTypes: true } })

console.log(stringify([1, 2, 3]))
// ACTUAL:   {}
// EXPECTED: [1,2,3]

Swapping the order to type: ['array', 'object'] produces the correct [1,2,3], so the bug is order dependent.

Root cause

buildMultiTypeSerializer emits one branch per listed type in list order. The object branch is generated by the default case as if (typeof input === "object" || input === null). Because typeof [] === 'object', that guard captures array input first, which makes the later else if (Array.isArray(input)) branch unreachable. The array is then run through the object serializer, which emits only declared properties and returns {}.

Fix

Add an explicit object case that excludes arrays from the object branch, mirroring the object guard already used in buildArrayTypeCondition:

if ((typeof input === "object" && !Array.isArray(input)) || input === null) { ... }

A sibling array type in the same type list can now match array input. Plain objects, toString objects for ['object', 'string'], and ['object', 'null'] null handling are unchanged, since none of those are arrays.

Authority

Round-trip invariant: JSON.parse(serialize(x)) must equal a schema-valid x. [1,2,3] is valid for the array member yet serialized to {}, breaking the invariant.

Tests

Added to test/typesArray.test.js: array input under type: ['object', 'array'] serializes to [1,2,3] (object input still serializes as an object), and a JSON:API-style data field with type: ['object', 'array'] round-trips an array of objects. Both fail on the current base (array serialized as {}) and pass with the fix.

Full suite: 475/475 passing.