Check that aggregate is only called on anndata

[ivirshup]

• Closes Return type of aggregate is inconsistent #2930
• Tests included or not required because:

• Release notes not necessary because: Fixing something that never made it to a release

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 73.14%. Comparing base (e6c7251) to head (3633712).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2950      +/-   ##
==========================================
- Coverage   75.48%   73.14%   -2.35%     
==========================================
  Files         116      116              
  Lines       12904    12872      -32     
==========================================
- Hits         9741     9415     -326     
- Misses       3163     3457     +294     

Files	Coverage Δ
scanpy/get/_aggregated.py	94.73% <90.00%> (-0.62%)	⬇️

... and 27 files with indirect coverage changes