Add types for merge.py

[flying-sheep]

• Closes #
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.

Project coverage is 83.81%. Comparing base (9838253) to head (b2160c3).
Report is 76 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/_core/merge.py	85.71%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1859      +/-   ##
==========================================
- Coverage   86.25%   83.81%   -2.45%     
==========================================
  Files          41       41              
  Lines        6288     6291       +3     
==========================================
- Hits         5424     5273     -151     
- Misses        864     1018     +154     

Files with missing lines	Coverage Δ
src/anndata/_core/merge.py	82.89% <85.71%> (-11.10%)	⬇️

... and 7 files with indirect coverage changes

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