Issue with aggregation when `.attrs` is replaced instead of `.attrs['transform']`

[LucaMarconato]

• I have found a weird bug with transformations (unrelated to this PR, but spotted during debugging), you can see the comment in https://github.com/scverse/spatialdata/blob/d10a42e0a349320343431083d785f710d2324dc5/src/spatialdata/_core/operations/transform.py#L438
• [x] I will open a separate issue to track it.