fix: preserve nested arrays in cloneDataFromOriginalDoc

[Nahid-NHB]

What?

cloneDataFromOriginalDoc mangles a json field's array-of-arrays value (e.g. [[1, 2], [3, 4]]) into an array of index-keyed objects ([{"0": 1, "1": 2}, {"0": 3, "1": 4}]) whenever the field is omitted from a partial update/PATCH and its value is carried forward from the original document.

Why?

cloneDataFromOriginalDoc shallow-clones each row of an array value with {...row}. typeof row === 'object' is also true for arrays, so when a row is itself an array (nested arrays, as opposed to the usual array/blocks-field row objects), the object-spread branch runs instead and turns it into an index-keyed object. On a publish operation this trips field validate and is caught, but draft saves skip validation, so the corrupted shape silently persists to the database. Downstream consumers that iterate the tuples then crash with TypeError: object is not iterable.

How?

Check Array.isArray(row) before the generic object check, and clone array rows with [...row] instead of {...row}, preserving their shape. This keeps the same one-level-deep shallow-clone semantics the function already uses for object rows — only the branching changes.

Testing performed

• Added a unit test (cloneDataFromOriginalDoc.spec.ts) covering nested-array cloning, non-mutation of the original, and the existing object/primitive clone behavior. Verified it fails without the fix and passes with it.
• Added an integration regression test in test/fields/int.spec.ts reproducing the exact reported flow: create a doc with a json field holding an array-of-arrays, then partially update the doc without touching that field, and assert the stored value is unchanged. Verified it fails without the fix (reproducing the reported corruption exactly) and passes with it.
• Ran the full test/fields and test/fields-relationship integration suites (sqlite) — all passing, no regressions.
• tsc --noEmit on packages/payload — no errors.
• prettier --check and eslint on all changed files — clean.

Fixes #17475