client 使用 snappy 压缩时，server 能正常解压，但是解压出来的数据和原始数据不一致。

**Describe the bug**
client 侧为了节省带宽，使用了 snappy 压缩，但是测试下来，发现服务端用 snappy 解压的数据和 client 侧输入的数据不一致。

**To Reproduce**
把测试用的请求 dump 到磁盘，然后用 SampleIterator 遍历，并分别用 brpc 中的 snappy 和 github 上的 snappy 解压。其中 brpc 里面的 snappy 每次解压的结果都不一致，而 github 仓库中的 snappy 每次解压的结果都是一致的。

```
brpc::SampleIterator iter(dir_);
while (true) {
std::unique_ptr sampled(iter.Next());
if (sampled == nullptr) {
break;
}

if (sampled->meta.compress_type() != brpc::COMPRESS_TYPE_SNAPPY) {
continue;
}

std::cout << "----------------- " << sampled->request.size() << "--------------" << std::endl;

butil::IOBuf brpc_decompressed_v1;
butil::IOBuf brpc_decompressed_v2;
ABSL_CHECK(brpc::policy::SnappyDecompress(sampled->request, &brpc_decompressed_v1));
ABSL_CHECK(brpc::policy::SnappyDecompress(sampled->request, &brpc_decompressed_v2));

if (brpc_decompressed_v1.to_string() == brpc_decompressed_v2.to_string()) {
std::cout << "brpc decompress, same" << std::endl;
} else {
std::cout << "brpc decompress, diff" << std::endl;
}

std::string snappy_uncompressed_v1;
std::string snappy_uncompressed_v2;
auto compressed = sampled->request.to_string();
ABSL_CHECK(snappy::Uncompress(compressed.data(), compressed.size(), &snappy_uncompressed_v1));
ABSL_CHECK(snappy::Uncompress(compressed.data(), compressed.size(), &snappy_uncompressed_v2));
if (snappy_uncompressed_v1 == snappy_uncompressed_v2) {
std::cout << "snappy decompress, same" << std::endl;
} else {
std::cout << "snappy decompress, diff" << std::endl;
}
}
```

**Expected behavior**

**Versions**
OS: ubuntu 20.04
Compiler: clang-18
brpc: 1.13.0
protobuf: 29.4

**Additional context/screenshots**

测试程序输出的结果。可以看到，brpc 里面的 snappy decompress 的结果，会有不一致的情况。而 github 上的 snappy，每次解压的结果都是一致的。

```
----------------- 105486--------------
brpc decompress, diff
snappy decompress, same
----------------- 47149--------------
brpc decompress, same
snappy decompress, same
----------------- 79794--------------
brpc decompress, diff
snappy decompress, same
----------------- 74139--------------
brpc decompress, same
snappy decompress, same
----------------- 107280--------------
brpc decompress, diff
snappy decompress, same
```

Start with brpc::policy::SnappyDecompress and reproduce the issue using the dumped requests and SampleIterator. Compare repeated decompression with snappy::Uncompress, then trace the brpc decompression path to identify why results differ. Done means repeated brpc decompression matches the original input and the standard Snappy result.

Written by the indexing model from the issue text.