Implement range reads tests

I think we may want to have our own tests for this backend. Even though `lsm-tree` has a comprehensive test suite, I feel like the code in this file is involved enough to at least have some lightweight testing.

For example, the range read seems not to be covered by the `LedgerDB`-level test suite. I ran the tests with coverage as follows:

```
cabal test storage-test --enable-coverage --test-options "-p LSM"
...
Test suite storage-test: RUNNING...
ouroboros-storage
Storage
LedgerDB
StateMachine
LSM: OK (7.35s)
+++ OK, passed 1000 tests.
...
```

And the coverage report (see attached, specifically the `Ouroboros.Consensus.Storage.LedgerDB.V2.LSM` module) seems to indicate that the range read was never executed (`hpc` highlights such code in yellow):

that agrees with the LedgerDB-level model actions.

[hpc_index.html.txt](https://github.com/user-attachments/files/21349573/hpc_index.html.txt)

_Originally posted by @geo2a in https://github.com/IntersectMBO/ouroboros-consensus/pull/1572#discussion_r2219435268_