disk_table snapshot test

**Describe the feature you'd like**

add ut for disk-table snapshot

**Additional context**

`src/storage/snapshot_test.cc`

The test file is named: src/storage/snapshot_test.cc. Look at existing tests in that file to understand the pattern for snapshot tests. You'll need to understand the disk-table component and how snapshots work in OpenMLDB. The goal is to add a unit test for the disk-table snapshot functionality.

索引モデルが issue の本文から書いたものです。