GH-50351: [C++] Fix KeyValueMetadata::DeleteMany crash with duplicate…

[AdvancedUno]

Rationale for this change

KeyValueMetadata::DeleteMany crashes (out-of-bounds write) when the indices vector
contains duplicates, e.g. DeleteMany({0, 0}). The compaction loop increments its shift
counter once per entry, so duplicates cause writes at negative offsets.

Additionally, out-of-range indices were only checked by DCHECK, so in release builds
DeleteMany({size}) silently deleted the last element and negative indices were UB.

What changes are included in this PR?

• DeleteMany now validates indices after sorting and returns Status::IndexError for any index out of [0, size), matching the behavior and message format of Delete. Metadata is left unmodified on error.
• Duplicate indices are removed with std::unique before compaction, so deleting the same index twice is equivalent to deleting it once.

Are these changes tested?

Yes, I added cases to KeyValueMetadataTest.Delete covering duplicate indices ({0,0}, {1,3,1,3,1}), out-of-bounds positive and negative indices (exact error messages), and that metadata is unchanged after a failed call.

Are there any user-facing changes?

Yes, minor behavioral fix: DeleteMany now returns IndexError for out-of-range indices
instead of crashing (duplicates) or silently corrupting state (release builds). No API change.

• GitHub Issue: [C++] KeyValueMetadata::DeleteMany method crashes with duplicate 0 indexes #50351

[pitrou]

I don't think we should so such extensive validation of input arguments here. Rather, we should document the pre-conditions when calling DeleteMany.