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.

从 DecimalVector.setBigEndian 和 Decimal256Vector.setBigEndian 开始，然后对比 setBigEndianSafe；检查固定宽度槽位和长度验证是如何处理的。为非零槽位复现一个超大输入，并添加覆盖测试，证明相邻内存保持不变，同时无效长度仍会抛出异常。完成标准是三个入口点都会在写入之前拒绝超大值。

由索引模型根据 Issue 内容生成。