Return chainable ObjectSchema from extend

[aouxwoux]

Checklist

• run npm run test and npm run lint
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin and the Code of conduct

Description

ObjectSchema.extend() currently returns a reduced object containing only a few schema methods. That keeps valueOf() working, but breaks normal fluent chaining such as .prop(), .without(), or .only() after extending a schema.

This changes extend() to return the full ObjectSchema instance created from the merged state, preserving the existing merged output while keeping the builder API chainable.

Fixes #249

Notes

No docs update was needed; this restores the expected fluent behavior described by the existing API.

Tests

• node --test test/ObjectSchema.test.js
• pnpm exec eslint lib/ObjectSchema.js test/ObjectSchema.test.js
• pnpm exec c8 --100 node --test
• pnpm exec tstyche