fix: write iospec directly with sparse dataset

[ilan-gold]

• See added test_write_dispatched_csr_dataset for reproducer of the underlying bug - there was no io-spec in the CS{R,C}Dataset writer, and was instead appended at the end. I am not sure why this was done but it prevents creating dispatched write functions properly on sparse datasets unless you check the type of the elem @ivirshup any history to this? It's been in since Specs for all elements #554
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.55%. Comparing base (5080856) to head (4264c8f).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2134      +/-   ##
==========================================
- Coverage   85.70%   85.55%   -0.15%     
==========================================
  Files          46       46              
  Lines        7078     7076       -2     
==========================================
- Hits         6066     6054      -12     
- Misses       1012     1022      +10     

Files with missing lines	Coverage Δ
src/anndata/_io/specs/methods.py	90.05% <100.00%> (-0.04%)	⬇️

... and 2 files with indirect coverage changes

[flying-sheep]

I can’t think of any reason why it would be like that 🤷