[Bug]: File-per-key storage fails to initialize when eviction is disabled

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## Bug Report

On current `main` , the file-per-key backend cannot complete `FileStorage::Init()` when `MOONCAKE_OFFLOAD_ENABLE_EVICTION=false`.

The call order is:

1. `storage_backend_->Init()`
2. `FileStorage::IsEnableOffloading()`
3. `storage_backend_->ScanMeta()`

For `StorageBackendAdaptor`, `IsEnableOffloading()` returns `INTERNAL_ERROR` while `meta_scanned_` is false when eviction is disabled. The first metadata scan is later in `FileStorage::Init()`, so initialization returns before it can ever establish the counters required by `IsEnableOffloading()`.

The adaptor unit test already documents that `ScanMeta()` must run before `IsEnableOffloading()` in this configuration, but the production initialization path does the reverse.

## Expected behavior

File-per-key storage should initialize successfully with eviction disabled, reconstruct existing usage before enabling new offloads, and register recovered LOCAL_DISK metadata with the master.

### Before submitting...

- [x] Searched existing issues and open PRs for this initialization failure.

Start at FileStorage::Init() and compare its call order with the StorageBackendAdaptor unit test, which documents that ScanMeta() must precede IsEnableOffloading() when eviction is disabled. Verify initialization with MOONCAKE_OFFLOAD_ENABLE_EVICTION=false, including usage reconstruction and registration of recovered LOCAL_DISK metadata with the master.

Written by the indexing model from the issue text.