fix(ipc): correct skip_field handling for V4 Union

[pchintar]

Which issue does this PR close?

• Closes Incorrect buffer skipping for V4 Union types in IPC skip_field #9828 .

Rationale for this change

Currently, skip_field does not correctly handle the buffer layout of Union types for V4 IPC.

In V4:

• Union includes a null buffer + type_ids (+ offsets for dense)

In V5:

• Union has no null buffer, only type_ids (+ offsets for dense)

create_array correctly handles this difference using a version check. However, skip_field always assumes a null buffer and does not skip type_ids, leading to buffer misalignment when skipping a Union column in V4.

This can cause incorrect decoding or runtime errors for projected columns.

What changes are included in this PR?

• Updated skip_field in arrow-ipc/src/reader.rs to:

  • conditionally skip the null buffer only for V4
  • explicitly skip the type_ids buffer
  • correctly handle dense vs sparse offsets

• Aligns skip_field behavior with create_array and actual IPC layout

Are these changes tested?

Yes.

• Added test: test_projection_skip_union_v4

• The test:

  • writes IPC data using V4 metadata
  • includes a Union column followed by an Int32 column
  • reads only the second column (skipping the Union)
  • verifies the output matches expected values

• The test fails before the fix and passes after

• All existing arrow-ipc tests pass (cargo test -p arrow-ipc --lib)

Are there any user-facing changes?

No.

[pchintar]

cc @alamb