fix(memory-storage): store snapshot in DatasetMemoryEntry.update

[AmariahAK]

DatasetMemoryEntry.update() stores a JSON snapshot of the provided
data instead of the direct object reference. Previously, when the same
mutable object was pushed multiple times with mutations between pushes,
all stored entries reflected the final mutated state because they
shared the same reference.
This is inconsistent with the file-system backed implementation
(DatasetFileSystemEntry), which serializes to JSON on write and
parses on read — naturally creating independent copies.
The fix changes this.data = data to
this.data = JSON.parse(JSON.stringify(data)) in
packages/memory-storage/src/fs/dataset/memory.ts.
A test (dataset-snapshot.test.ts) verifies that each push creates an
independent snapshot by pushing a mutable object three times with
mutations between pushes and asserting each stored entry preserves its
push-time value.
Related: #3809 (stale references held in memory may prevent GC — the
snapshot approach helps by breaking reference chains, though that issue
is not fully resolved by this change alone).
Closes #3812

[barjin]

Thank you for your interest in this project @AmariahAK !

The changes you are proposing are noop - by the time the data reaches the DatasetMemoryEntry.update method, it's been already serialized by the Dataset.pushData method ⬇️

crawlee/packages/core/src/storages/dataset.ts

Line 285 in 15f9e55

const payload = checkAndSerialize(data, limit);

See that the original issue is for the v4 branch, where the storage management has been revamped. The issue doesn't present in the current master (v3).

Closing this PR, feel free to open another one against the v4 branch, if you feel like it.

[AmariahAK]

@barjin , i made a fix to this pr #3831