[maxymlyskov]

Summary

Fixes #16618.

push(3) then push({ $each: [4] }) before a save sends only { $push: { nums: { $each: [4] } } }, so the 3 never reaches the database. _registerAtomic() replaces a pending $push whenever the new one has $each, and push() only falls back to $set for two different $position values (gh-14244). Same with two { $each } pushes. The other order breaks too: a plain push(3) after push({ $each: [0], $position: 0 }) joins the positioned $each and is saved at the front.

Now a second push() adds its values to the pending $each instead of replacing it. If either push has $position, $slice or $sort, the whole array is saved with $set, like the different $position case already is.

One catch: if a $push is already pending, a $slice or $sort on either push isn't sent, because the array is saved as the document holds it. A single push still sends them. I didn't touch a plain push() after a pending $push with $slice or $sort, it still joins that $push like on master.

Examples

const doc = await Test.create({ nums: [1, 2] });
doc.nums.push(3);
doc.nums.push({ $each: [4] });
await doc.save();
// before: db has [1, 2, 4]
// after: db has [1, 2, 3, 4]

const doc2 = await Test.create({ nums: [1, 2] });
doc2.nums.push({ $each: [0], $position: 0 });
doc2.nums.push(3);
await doc2.save();
// before: db has [0, 3, 1, 2]
// after: db has [0, 1, 2, 3], saved with $set

• Ran: npx mocha test/types.array.test.js -g "gh-16618". The 3 new tests (one on a document array) fail on master and pass here. test/document.test.js still passes, different behavior of arrays in 6.x when compared to 5.x #14244 included.
• Ran: npm run test:ci, 4677 passing, 47 pending, 0 failing.
• npx eslint on the changed files is clean.

Node 22.23.3, MongoDB 8.2.6 (mongodb-memory-server), Linux.