Fix sparse dataset 2D slicing

[flying-sheep]

• [x] Closes #1520
• [x] Tests added
• [ ] Release note added (or unnecessary)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.41%. Comparing base (f87f3a8) to head (2b188e3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1523 +/- ## ========================================== - Coverage 84.43% 84.41% -0.03% ========================================== Files 36 36 Lines 5745 5743 -2 ========================================== - Hits 4851 4848 -3 - Misses 894 895 +1 ``` | [Files](https://app.codecov.io/gh/scverse/anndata/pull/1523?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse) | Coverage Δ | | |---|---|---| | [src/anndata/\_core/sparse\_dataset.py](https://app.codecov.io/gh/scverse/anndata/pull/1523?src=pr&el=tree&filepath=src%2Fanndata%2F_core%2Fsparse_dataset.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL2FubmRhdGEvX2NvcmUvc3BhcnNlX2RhdGFzZXQucHk=) | `92.92% <100.00%> (-0.35%)` | :arrow_down: |

[flying-sheep]

You mean another test parameter set or something else utilizing the helpers?

[ilan-gold]

You mean another test parameter set or something else utilizing the helpers?

Yes, something besides [0] as the idx