Azure storage on Windows: checkpoint and AOF commit base names embed backslashes, breaking container/blob naming

Describe the bug

When Garnet runs on Windows with DeviceType.AzureStorage, the base names handed to AzureStorageNamedDeviceFactory for checkpoints and AOF commits are built with Path.Combine, which inserts a backslash. Azure treats / as its blob hierarchy separator and parses the base name with Split('/'), so the backslash is not a separator there.

Two base names are affected:

Both resolve through:

public string StoreCheckpointBaseDirectory => Path.Combine(CheckpointBaseDirectory, "Store");
public string GetStoreCheckpointDirectory(int dbId) =>
    Path.Combine(StoreCheckpointBaseDirectory, GetCheckpointDirectoryName(dbId));
public string GetAppendOnlyFileDirectory(int dbId) =>
    Path.Combine(AppendOnlyFileBaseDirectory, GetAppendOnlyFileDirectoryName(dbId));

Options.GetServerOptions() only canonicalizes checkpointDir for non-Azure configs (if (!useAzureStorage) checkpointDir = new DirectoryInfo(...).FullName;), so with Azure the user-supplied value flows through as-is and the Path.Combine calls above append \ on Windows.

AzureStorageNamedDeviceFactory then does:

var path = baseName.Split('/');
var containerName = path[0];
var dirName = string.Join('/', path.Skip(1));

The impact depends on the form of --checkpointdir:

On Linux Path.Combine yields /, so the composition happens to be correct there and this is Windows-only.

Steps to reproduce the bug

1. On Windows, start Garnet configured for Azure storage — --storage-string <connection-string> with a device type of AzureStorage — and set --checkpointdir mycontainer.
2. Trigger a checkpoint (SAVE / BGSAVE).
3. The container name is rejected because it contains backslashes. With --checkpointdir mycontainer/myprefix instead, the operation succeeds but the blobs are created under a literal myprefix\Store\checkpoints name rather than the nested hierarchy.

Expected behavior

Base names handed to a device factory should stay backend-neutral, and path segments below them should be expressed via FileDescriptor.directoryName so each factory composes them with its own separator — Path.Combine for LocalStorageNamedDeviceFactory, GetSubDirectory (which joins with /) for AzureStorageNamedDeviceFactory.

Additional context

There is already a correct precedent in the same file — the AOF log device does exactly this:

// GarnetServerOptions.cs:1223
return GetInitializedDeviceFactory(AppendOnlyFileBaseDirectory)
    .Get(new FileDescriptor(GetAppendOnlyFileDirectoryName(dbId), "aof.log"));

AppendOnlyFileBaseDirectory is CheckpointDir ?? string.Empty with no Path.Combine, and the per-database subdirectory is supplied through the descriptor. Only the two DefaultCheckpointNamingScheme base names above still embed segments via Path.Combine.

The same pattern was applied to the cluster config paths in #2147 (review feedback on ClusterManager / ReplicationManager), which is what surfaced this. That PR deliberately does not change GarnetServerOptions, so these two sites are unaffected by it and remain as they are on main.

Note that a fix changes the on-disk/blob layout for existing Windows+Azure deployments in the container/prefix case, since data previously written under the backslash name would no longer be found — worth considering whether a migration or a release note is needed.