Store the relative path instead of absolute path in `KvFileHandle`

Currently, we will store the absolute path in `KvFileHandle`, something like:
```
ossj://fluss-xxx/remote-dir/kv/fluss_db/flusstable-0/20250304-p8/105/snap-64
```
Actual, we can just store `snap-64` in `KvFileHandle`, and make `ossj://fluss-xxx/remote-dir/kv/fluss_db/flusstable-0/20250304-p8/105` stored in `CompletedSnapshot`

Start by locating KvFileHandle and CompletedSnapshot, then trace how their paths are created, stored, and read. Confirm how the relative snapshot name and CompletedSnapshot base path are combined; done means KvFileHandle stores only the relative name while the base path remains in CompletedSnapshot, with existing snapshot behavior preserved.

Written by the indexing model from the issue text.