[Java] splitAndTransfer of a view vector leaves non-zero padding in inlined views #1386

Describe the bug, including details regarding any error messages, version, and platform.

Disclaimer: Claude Opus 5 found this bug.

Describe the bug

BaseVariableWidthViewVector.splitAndTransferViewBufferAndDataBuffer allocates the target's view
buffer without zeroing it:

target.viewBuffer = target.allocator.buffer(length * ELEMENT_SIZE);  // line 888 at v19.0.0

For an inlined value (12 bytes or fewer) it then writes only the 4-byte length and the value's
bytes, so the rest of the 16-byte view keeps whatever the allocation held. The Arrow columnar
format requires those padding bytes to be zero, and setBytes already does this (it zeroes the
view before writing an inlined value), but this path does not.

So any splitAndTransfer of a ViewVarCharVector / ViewVarBinaryVector can produce invalid
views. That includes TransferPair.splitAndTransfer and VectorSchemaRoot.slice(). Consumers
that validate are then right to reject the data. For example, after export through the C Data
Interface, arrow-rs's ArrayData::validate_full fails with:

View at index 0 contained non-zero padding for string of length 0

To reproduce

import java.nio.charset.StandardCharsets;
import org.apache.arrow.memory.ArrowBuf;
import org.apache.arrow.memory.BufferAllocator;
import org.apache.arrow.memory.RootAllocator;
import org.apache.arrow.vector.ViewVarCharVector;
import org.apache.arrow.vector.util.TransferPair;

public class PaddingRepro {
    public static void main(String[] args) {
        try (BufferAllocator allocator = new RootAllocator();
             ViewVarCharVector v = new ViewVarCharVector("s", allocator)) {
            v.allocateNew();
            v.setSafe(0, "a longer value".getBytes(StandardCharsets.UTF_8));
            v.setSafe(1, "".getBytes(StandardCharsets.UTF_8));
            v.setValueCount(2);
            TransferPair tp = v.getTransferPair(allocator);
            tp.splitAndTransfer(1, 1);
            ArrowBuf views = ((ViewVarCharVector) tp.getTo()).getDataBuffer();
            for (int i = 4; i < 16; i++) {
                System.out.print(views.getByte(i) + " ");   // expected all 0
            }
            System.out.println();
            tp.getTo().close();
        }
    }
}

With arrow-vector 19.0.0 on HotSpot 21 this prints (the exact bytes vary):

-112 127 0 0 -112 0 0 8 -112 127 0 0

Expected behavior

All 12 bytes after the length of an inlined view are zero after splitAndTransfer, for example
by zeroing the target's view buffer after allocating it (setZero), as setBytes does for a
single view. Present at v19.0.0 and unchanged on main.

Component(s)

Java (arrow-vector 19.0.0)