fix(queryCursor): add _transformForAsyncIterator transform after user-defined transforms

[vkarpov15]

Fix mongoosejs/mongoose-lean-getters#44

Summary

mongoosejs/mongoose-lean-getters#44 (comment) pointed out an issue which is caused by how we handle async iterators in queryCursor.js. Query cursors use a _transformForAsyncIterator transform which converts doc to { value: doc, done: false } or { done: true } depending on whether the cursor returned null. Because transforms pass the transformed value to the next transform, _transformForAsyncIterator breaks user-specified transforms if it gets added first.

This PR makes it so that user-specified transforms are applied before _transformForAsyncIterator, which means user-specified transforms won't see the { value: doc, done: false } doc.

However, I want to do some further work to check if we can make _transformForAsyncIterator not rely on transforms. transformNull() also adds a weird edge case for user-specified transforms where user-specified transforms need to handle null if using cursors. Which is why the test added in this PR has doc?.name.toUpperCase() ?? null.

Examples

[Copilot]

Pull Request Overview

This PR fixes an issue with async iterator transforms by ensuring user-defined transforms are applied before the internal _transformForAsyncIterator.

• Reorders the addition of transforms in QueryCursor so that user transforms are pushed before the async iterator transform.
• Adds a new test case in query.test.js to verify correct transform handling with for/await loops.

Reviewed Changes

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

File	Description
test/query.test.js	Adds a test to check that user-specified transforms work correctly in async iteration
lib/cursor/queryCursor.js	Reorders and flags the internal async iterator transform to run after user-specified transforms

[Copilot]

Pull Request Overview

This PR fixes an issue with user-defined transforms in async iterators by applying them before the internal async iterator transform.

• Updates the async iterator implementation in queryCursor.js by removing transformNull and adjusting transform order.
• Adds a test case to ensure that transforms work correctly with async iteration.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
test/query.test.js	New test added for async iterator transform with user-defined transform.
lib/cursor/queryCursor.js	Removed transformNull and updated async iterator implementation to flag async iteration.

[vkarpov15]

@hasezoey @AbdelrahmanHafez I changed my implementation to not rely on transforms, please take another look 👍

[hasezoey]

Looks good to me, though some other parts need to be updated to fix the tests.

Also somewhat related, i noticed that asyncIterator is conditionally assigned because of old nodejs versions (<10.0), should we maybe remove that condition for 9.x to reduce the conditions?
(If the comments are anything to go by, then it could be done for 8.x, though just for accidental breakage i would recommend for 9.x)