`from_ffi` realignment is bypassed under `force_validate`

[mbutrovich]

Follow-up to #10030 / #10028.

Describe the bug

arrow::ffi::from_ffi and from_ffi_and_data_type call data.align_buffers() after ImportedArrowArray::consume() to realign protocol-legal but under-aligned C Data Interface buffers (e.g. 8-byte-aligned Decimal128 from a JVM producer). Under cfg(feature = "force_validate") this realignment is unreachable: consume() constructs ArrayData via ArrayData::new_unchecked, which goes through ArrayDataBuilder::build. That builder runs validate_data() whenever force_validate is enabled, regardless of skip_validation:

// arrow-data/src/data.rs:2195
if !skip_validation.get() || cfg!(feature = "force_validate") {
    data.validate_data()?;
}

validate() rejects misaligned BufferSpec::FixedWidth buffers (arrow-data/src/data.rs:842-867), so consume()? returns InvalidArgumentError before from_ffi's align_buffers() runs. The same applies recursively through consume_children / consume_child.

The new regression test added in #10030 (test_decimal128_under_aligned_round_trip) is gated #[cfg(not(feature = "force_validate"))], so CI under that feature does not surface the gap.

This is not a regression introduced by #10030 — prior to that PR, force_validate builds also returned an error rather than panicking. But the inline comment "a no-op when buffers are already aligned" reads as if alignment is handled in all builds, which is misleading.

To Reproduce

Build with --features force_validate and run the round-trip with an 8-byte-aligned-but-not-16-byte-aligned Decimal128 buffer through from_ffi. consume() returns InvalidArgumentError before realignment can run.

Expected behavior

from_ffi accepts spec-conformant C Data Interface input regardless of force_validate. 8-byte alignment is legal over the C Data Interface; it only becomes an arrow-rs ArrayData invariant violation after import. Today the order is "validate, then realign", which treats protocol-legal input as an invariant violation. The correct order on the import boundary is "realign, then validate".

Proposed fix

Push alignment into ImportedArrowArray::consume (and recursively through consume_child) by replacing ArrayData::new_unchecked with the builder form:

ArrayDataBuilder::new(self.data_type)
    .len(len)
    .offset(offset)
    .null_count(null_count)
    .null_bit_buffer(null_bit_buffer)
    .buffers(buffers)
    .child_data(child_data)
    .align_buffers(true)
    .skip_validation(true)
    .build()?

ArrayDataBuilder::build runs align_buffers before validate_data (arrow-data/src/data.rs:2190-2197), so this works correctly under force_validate while preserving the existing intent of skipping FFI validation on the hot path.

Then:

• Drop the now-redundant outer data.align_buffers() calls in from_ffi / from_ffi_and_data_type to avoid a duplicate recursive scan.
• Remove the #[cfg(not(feature = "force_validate"))] gate on test_decimal128_under_aligned_round_trip so the case is covered under that feature too.

It would also be worth tightening the doc on ArrayData::align_buffers to note that on import boundaries it must be called before validation; the current doc only says it's "useful when interacting with buffers produced by other systems" without addressing ordering.

Additional context

Raised in review of #10030: #10030 (review)

[bit2swaz]

im taking this. ill push the align into consume()'s builder so it runs before the force_validate-gated validate (same as the ipc reader) and drop the now-redundant outer align_buffers(). brb