fix: do not let a string type swallow array input in multi-type schemas

[chuanghiduoc]

When a schema lists string before array in its type array, array input serializes to a comma-joined string and the JSON structure is lost.

Steps to reproduce

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

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

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

Nested objects are destroyed the same way:

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

stringify([{ a: 1 }, { a: 2 }])
// ACTUAL:   "[object Object],[object Object]"
// EXPECTED: [{"a":1},{"a":2}]

Swapping the order to type: ['array', 'string'] produces the correct output, so the bug is order dependent.

Root cause

buildMultiTypeSerializer emits one branch per listed type, in list order. The string branch accepts anything that quacks like a stringifiable object:

typeof input === "object" &&
typeof input.toString === "function" &&
input.toString !== Object.prototype.toString

An array satisfies all three (Array.prototype.toString !== Object.prototype.toString), so array input is captured by the string branch and the later else if (Array.isArray(input)) branch is unreachable.

This is the same class of bug as #851 (object branch capturing arrays because typeof [] === 'object'), which was fixed by excluding arrays from that branch.

Fix

Exclude arrays from the string branch's duck-typing check, mirroring the object-branch guard from #851. Date/RegExp and genuine custom-toString objects keep working, since they are matched by the earlier instanceof checks or still pass the (now array-excluded) object check.

Tests

Added multi-type [string, array] round-trips an array to test/typesArray.test.js, covering both the array input and a plain string input for the same schema. It fails on main (fail 1) and passes with the fix.

Full unit suite: 501 passing, 0 failing, coverage stays at 100%.