fix: ObjectSchema.extend returns a schema instance

[deepaknf]

Summary

This PR addresses the issue #249 where ObjectSchema.extend Does not return a schema instance. The fix makes it return the schema.

Checklist

• run npm run test
• 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

[bencoder]

It seems it was intentionally not returning a Schema instance before this. Do we know why it was like that in the first place?

Looking at where that was originally changed (to not return Schema) I see this PR & issue with some explanation but I don't understand the context well enough to say if it's still valid:

#69
#65

[kieranswhite]

@aboutlo I don't suppose you can help us understand this? It looks like you were heavily involved in the previous issues that @bencoder listed. Thanks :)

[simoneb]

I don't think we can safely merge this change, precisely because, as observed, this behavior is intentional. The change is straightforward, but before we even change anything, we need to understand why the current behavior was intentionally implemented in this way, and the explanation lies in the PRs linked by @bencoder above. So, please have a look at the whole history before deciding what to do with this issue. A viable option is to close it as it works as designed, but I would consider giving another look at the original motivations behind this behavior, to see if there's a solution that wasn't considered back then.

[Copilot]

Pull Request Overview

This PR addresses an issue where ObjectSchema.extend did not return a schema instance, causing unexpected chaining problems. The changes include:

• Updating ObjectSchema.extend to return a complete fluent schema instance.
• Adding a new test to verify chainability with the without() method.
• Removing the test that expected an error when calling prop() after extend.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/ObjectSchema.test.js	Introduces a test verifying chaining after extend works with without().
src/ObjectSchema.js	Modifies extend() so that it returns the full schema instance, enabling seamless chaining.

Comments suppressed due to low confidence (1)

src/ObjectSchema.js:343

• Returning a full schema instance from extend() improves chainability; please verify that upstream documentation and consumers are updated to reflect this API change.

return ObjectSchema({ schema: extended, ...options })

[esatterwhite]

@aboutlo are you able to chime in on this?