fix(documentarray): reindex subdocs after reordering

[AbdelrahmanHafez]

Document array operations that remove or reorder entries can leave surviving subdocs with stale __index values. After operations like pull(), shift(), splice(), $shift(), unshift(), positioned push(), sort(), or reverse(), a subdoc can still think it lives at its previous array index. A later nested change then marks the wrong path, and since the delta serializes the value from whatever subdoc currently sits at that stale index, the update writes a no-op to the wrong element and the user's change is silently lost:

const user = await User.create({
  addresses: [{ city: 'Boston' }, { city: 'Chicago' }, { city: 'Denver' }]
});
user.addresses.pull(user.addresses[0]._id);
await user.save();

user.addresses[0].city = 'New York';
user.$getChanges(); // { $set: { 'addresses.1.city': 'Denver' } }, 'New York' never reaches the DB

This PR:

• restamps document-array subdoc indexes after operations that can move existing subdocs: pull(), shift(), $shift(), splice(), unshift(), positioned push(), sort(), reverse()
• keeps append-only push() on the fast path, the new subdoc is already cast with the correct appended index and existing indexes can't move, so repeated appends don't become O(n²)
• refreshes the parent populated cache in operations that previously skipped it: $shift(), unshift(), sort(), reverse(), pop(), $pop()
• makes document-array reverse() register a full-array $set, matching sort()/splice()/unshift() and avoiding stale pending atomics after a prior $push
• adds regression coverage asserting the exact $getChanges() and modifiedPaths() after each operation, persistence after save, populated-cache order, and reverse atomics
• fixes the MDN links in the touched doc comments, shift() was pointing at the unshift article

addToSet(), nonAtomicPush(), and set() have the same populated cache gap, I'll send that as a follow-up PR once this one lands.

[Copilot]

Pull request overview

This PR fixes document array subdocument index bookkeeping after removals so later nested mutations mark the correct path.

Changes:

• Reindexes surviving subdocuments after pull(), shift(), and splice().
• Preserves populated cache updates in the same wrappers.
• Adds regression coverage for saving after removal and mutating the first remaining subdocument.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/types/documentArray/methods/index.js	Adds subdocument index restamping after selected document-array removal operations.
test/types.documentarray.test.js	Adds regression tests for pull(), splice(), and shift() reindex behavior.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

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

[hasezoey]

LGTM

[vkarpov15]

I spotted one issue to follow up on, but otherwise LGTM this is a meaningful improvement

[vkarpov15]

There's a related error with $setIndex(): $setIndex() adds new validation error keys, but retains existing ones. For example:

user.addresses[1].invalidate('city', 'invalid city');
user.addresses.sort((a, b) => a.sequence - b.sequence);
user.$__.validationError.errors; // Has both `addresses.1.city` and `addresses.0.city` presuming that the invalid city was sorted into position 0

[vkarpov15]

Gonna squash this to make it easier to backport to 8.x.