DecimalVector.setBigEndian writes oversized value before validating length

DecimalVector.setBigEndian(int, byte[]) and Decimal256Vector.setBigEndian(int, byte[]) validate the input length only after copying the bytes. On a little-endian platform the method byte-swaps the value into the fixed 16/32-byte slot with a loop of unchecked MemoryUtil.putByte writes:

```java
for (int byteIdx = 0; byteIdx < length; ++byteIdx) {
MemoryUtil.putByte(outAddress + byteIdx, value[length - 1 - byteIdx]);
}
...
throw new IllegalArgumentException("Invalid decimal value length. Valid length in [1 - 16], got " + length);
```

If value.length exceeds the type width, the loop writes past the slot into adjacent off-heap memory before the IllegalArgumentException is thrown. MemoryUtil.putByte does no bounds checking, so this corrupts memory in the default configuration. setBigEndianSafe(int, long, ArrowBuf, int) performs the same write with no length check at all.

Reproducer: write a value into slot 1, then call setBigEndian(0, new byte[24]) on a DecimalVector; slot 1 is left corrupted (the write spills 8 bytes into it) even though the call throws.

從 DecimalVector.setBigEndian 和 Decimal256Vector.setBigEndian 開始，然後比較 setBigEndianSafe；檢查固定寬度槽位和長度驗證是如何處理的。為非零槽位重現一個過大的輸入，並新增涵蓋測試，證明相鄰記憶體保持不變，同時無效長度仍會拋出例外。完成標準是三個入口點都會在寫入之前拒絕過大的值。

由索引模型根據 Issue 內容生成。