Fix: compact view buffers in ScalarValue::compact for all container t…

[bert-beyondloops]

Which issue does this PR close?

• Closes ScalarValue::compacted() does not free unused view-buffer memory for Utf8View / BinaryView arrays #21928.

Rationale for this change

ScalarValue::compact copies array data for container types to release sliced-buffer overhead, but never called .gc() on nested StringViewArray / BinaryViewArray. A scalar extracted from a large batch would therefore still hold a reference to the entire original view backing buffer, negating the benefit of compaction for any list, struct, or map whose values are view-typed.

What changes are included in this PR?

• ScalarValue::compact now compacts nested view buffers for all container types (List, LargeList, FixedSizeList, ListView, LargeListView, Struct, Map), trimming StringViewArray / BinaryViewArray backing buffers to only the
  referenced bytes.
• The internal compact_view_buffers helper recursively handles FixedSizeList, ListView, LargeListView, and Map.

Are these changes tested?

new unit tests are added in scalar::tests, one per container type. Each test verifies that after compact() the backing buffer is reduced to exactly the bytes of the referenced string, and that the scalar value is preserved.

Are there any user-facing changes?

No. The public compact / compacted API is unchanged; this PR only fixes the behaviour for view-typed nested arrays.

[bert-beyondloops]

I tried to come up with a minimal solution.
If the implementation should somehow reside in another place, or made in another way, please feel free to comment.