Fix concat_on_disk

[milos7250]

• Closes concat_on_disk fails to write alternative axis mapping and uns #1854
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.85%. Comparing base (7fc66bb) to head (8c3efb2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/experimental/merge.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1855      +/-   ##
==========================================
+ Coverage   83.84%   83.85%   +0.01%     
==========================================
  Files          41       41              
  Lines        6288     6292       +4     
==========================================
+ Hits         5272     5276       +4     
  Misses       1016     1016              

Files with missing lines	Coverage Δ
src/anndata/experimental/merge.py	87.67% <85.71%> (+0.23%)	⬆️