[Java] ArrowRecordBatch Body Length Computation Only Accounts for Last Buffer

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

Hi Arrow Community,

I believe `ArrowRecordBatch::computeBodyLength` has an implementation bug. It does the following

```
for (int i = 0; i < buffers.size(); i++) {
ArrowBuf buffer = buffers.get(i);
ArrowBuffer layout = buffersLayout.get(i);
size = layout.getOffset() + buffer.readableBytes();

// round up size to the next multiple of 8
size = DataSizeRoundingUtil.roundUpTo8Multiple(size);
}
return size;
```

Since `size` is being assigned and not incremented at first - does this mean computeBodyLength only accounts for the last buffer ?

### Component(s)

Java

Start at ArrowRecordBatch::computeBodyLength and inspect how buffer layouts and readable bytes contribute to the returned value. Reproduce the calculation with multiple buffers, then check the surrounding Java code or tests for the expected body-length semantics. Done means the computation accounts for every buffer and preserves the required 8-byte rounding.

Written by the indexing model from the issue text.