Fix a compilation error when running the following command line:
```
bazel build --build_atlas --opt=on --dbg=True --compiler_type=gcc //src/mongo/db/modules/atlas/src/disagg_storage/pali:db_pali_test
```

The error was that in [storage_engine_mock.h](https://github.com/10gen/mongo/blob/master/src/mongo/db/storage/storage_engine_mock.h#L91-L96), we tried to instantiate an unique_ptr with the incomplete type `SpillTable`.