[Safety] Filtering Dense UnionArray Produces Invalid Offsets

[tustvold]

Describe the bug

build_extend_dense blindly copies offsets from the source array across, which can lead to invalid offsets in the filtered array

To Reproduce

let mut builder = UnionBuilder::new_dense(4);
builder.append::<Int32Type>("A", 1).unwrap();
builder.append::<Float64Type>("B", 3.2).unwrap();
builder.append::<Float64Type>("B", 3.2).unwrap();
let array = builder.build().unwrap();

let filter_array = BooleanArray::from(vec![true, false, true]);
let c = filter(&array, &filter_array).unwrap();
let filtered = c.as_any().downcast_ref::<UnionArray>().unwrap();

filtered.value(1);

Panics with

thread 'compute::kernels::filter::tests::test_filter_union_array_dense_with_nulls' panicked at 'assertion failed: (offset + length) <= self.len()', arrow/src/array/data.rs:486:9
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Expected behavior

This should not panic