chore: add anndata.settings.copy_on_write_X

[ilan-gold]

copy_on_write_X will be False and then the setting will be removed in the next minor release (i.e., True).

• Grossly simplifies feat!: Unify X and layers #1707 because we will remove the implicit modification of X
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

❌ Patch coverage is 94.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.62%. Comparing base (26478f0) to head (5375787).
⚠️ Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/_core/anndata.py	93.87%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2327      +/-   ##
==========================================
- Coverage   86.72%   84.62%   -2.10%     
==========================================
  Files          46       46              
  Lines        7165     7182      +17     
==========================================
- Hits         6214     6078     -136     
- Misses        951     1104     +153     

Files with missing lines	Coverage Δ
src/anndata/_settings.py	92.85% <100.00%> (+0.04%)	⬆️
src/anndata/_core/anndata.py	83.68% <93.87%> (+0.33%)	⬆️

... and 8 files with indirect coverage changes

[ilan-gold]

Tested locally against scanpy and getting no warnings. I also ctrl+F'ed the codebase and don't see anywhere this might be triggered anyway.

[flying-sheep]

Awesome, very few code changes necessary!

Do please reduce complexity instead of adding more ignores for complexity warnings though.