Backport PR #3643 on branch 1.11.x ((fix): sc.read with extension does not warn unnecessarily)

[ilan-gold]

• (fix): sc.read with extension does not warn unnecessarily

• (fix): obey return_ext

• (fix): add extension check internally

• (chore): relnote

(cherry picked from commit 350c342)

• Closes #
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.83%. Comparing base (7c27a82) to head (16d9bab).
Report is 2 commits behind head on 1.11.x.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/readwrite.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           1.11.x    #3661      +/-   ##
==========================================
+ Coverage   74.82%   74.83%   +0.01%     
==========================================
  Files         115      115              
  Lines       13198    13207       +9     
==========================================
+ Hits         9875     9884       +9     
  Misses       3323     3323              

Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	75.51% <90.90%> (+0.51%)	⬆️