Bug: NewAppendableFile does not record newly created FileState in file_map_

## Issue Description

I found a potential bug in `helpers/memenv/memenv.cc` in the `InMemoryEnv::NewAppendableFile` method.

## Location
[helpers/memenv/memenv.cc, lines 276-287](https://github.com/google/leveldb/blob/main/helpers/memenv/memenv.cc#L276)

## Problem

When a new appendable file is created (i.e., the file does not exist in `file_map_`), a new `FileState` object is instantiated and referenced. However, **the new FileState is never assigned back to the file_map_**, breaking the association between the filename and the FileState object.

## Current Code

```cpp
Status NewAppendableFile(const std::string& fname,
WritableFile** result) override {
MutexLock lock(&mutex_);
FileState** sptr = &file_map_[fname];
FileState* file = *sptr;
if (file == nullptr) {
file = new FileState();
file->Ref();
// BUG: Missing assignment back to file_map_
}
*result = new WritableFileImpl(file);
return Status::OK();
}