fix: serialize BigInt values in integer anyOf/oneOf and multi-type schemas

[marko1olo]

Problem

When a schema field is typed as anyOf: [{type: 'integer'}, {type: 'null'}] or type: ['integer', 'null'], passing a BigInt value (e.g. 12n) throws an error instead of serializing it as an integer.

const stringify = build({ anyOf: [{ type: 'integer' }, { type: 'null' }] })
stringify(12n)
// Before: TypeError: The value of '#' does not match schema definition.
// After:  "12"

This was reported in #501 where Typebox's Type.Union([Type.Integer(), Type.Null()]) produces exactly this schema shape.

Root cause

Two code paths were affected:

1. buildMultiTypeSerializer (for type: ['integer', 'null']): the branch condition was Number.isInteger(x) || x === null. Number.isInteger(12n) returns false, so BigInt fell through.

2. buildOneOf (for anyOf/oneOf): branch selection uses validator.validate() (AJV). AJV does not recognise BigInt as type: 'integer' by default, so BigInt never matched any branch.

Note: lib/serializer.js asInteger() already handles BigInt correctly via .toString().

Fix

• buildMultiTypeSerializer: add typeof input === 'bigint' to the integer guard.
• buildOneOf: when an option schema has type: 'integer', prefix the AJV check with typeof input === 'bigint' || so BigInt values are routed to asInteger().

Tests

Added 2 new test cases in test/typesArray.test.js covering both code paths. Full 501-test suite passes.