fix(arrow/scalar): release partial children after failed struct conversion

[fallintoplace]

Rationale for this change

ToScalar creates child scalars incrementally. If a later Struct field fails, earlier array-backed children can remain retained after the conversion returns an error.

What changes are included in this PR?

Release partial children on failure while keeping the existing ownership transfer for a completed struct scalar. Add checked-allocator coverage for the failed conversion.

Are these changes tested?

• go test ./arrow/scalar

Are there any user-facing changes?

No API changes. This corrects the reported behavior while preserving the existing ownership and compatibility contracts.

[zeroshade]

@fallintoplace Heads-up: #1106 has just been merged into main. It touches the same arrow/scalar/ code as this PR, so GitHub now reports this branch as conflicting.

Nothing is wrong with this PR — it is still approved. It just needs a rebase onto the latest main to resolve the overlap, and then it can go in.

Apologies for the extra round-trip: the conflict is a side effect of the sibling fix landing first, not anything you did.

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