fix(model): don't mutate the caller's update object in bulkWrite updateMany - #16526

Summary

Model.bulkWrite() with an updateMany op mutates the update object the caller passed in.

castUpdateOne() clones the update before applying timestamps, defaults and the version key (added in #14164, "deep clone bulkWrite() updateOne arguments to avoid mutating documents in update"), but castUpdateMany() never received the same treatment: it runs setDefaultsOnInsert(), applyTimestampsToUpdate(), applyTimestampsToChildren() and decorateUpdateWithVersionKey() directly on updateMany.update.

const schema = new Schema({ name: String }, { timestamps: true });
const Test = db.model('Test', schema);

const update = { $set: { name: 'Renamed' } };
await Test.bulkWrite([{ updateMany: { filter: {}, update } }]);
console.log(update); // { $set: { name: 'Renamed', updatedAt: 2026-... } }  <- mutated

With updateOne instead of updateMany, update comes back unchanged. The gh-14164 scenario (update: { $set: doc }) likewise writes updatedAt into the caller's document for updateMany.

This PR clones the update in castUpdateMany() exactly the way castUpdateOne() does, and passes the clone to every helper; the cast result is still assigned back to updateMany.update.

Examples

Added should not modify the update object passed to updateMany (gh-14164) in test/model.test.js next to the existing gh-14164 test. It covers both the $set: doc shape and a plain update object, and checks the write itself still lands. It fails on master and passes with this change.