fix: ensure read_10x_mtx returns CSR format

[ernestprovo23]

Summary

Ensure read_10x_mtx() returns an AnnData object with a CSR sparse matrix instead of CSC.

Motivation

Fixes #4016

When _read_10x_mtx() transposes the matrix read by anndata.read_mtx(), scipy automatically converts the CSR matrix to CSC format (this is how scipy sparse transpose works — CSR.T → CSC). Most downstream scanpy operations expect CSR format.

Changes

• Add adata.X = adata.X.tocsr() after the .T transpose in _read_10x_mtx() (src/scanpy/readwrite.py)
• Add CSR format assertion to test_read_10x (tests/test_read_10x.py)

Testing

• All 4 test_read_10x parametrized cases pass locally (v1.2.0 + v3.0.0, with and without prefix)
• New assertion verifies the matrix is CSR after reading

Notes for reviewers

• First contribution to scanpy
• Single-line fix + test assertion, minimal scope

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.34%. Comparing base (2ddfcc7) to head (cef15c0).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4017   +/-   ##
=======================================
  Coverage   78.34%   78.34%           
=======================================
  Files         117      117           
  Lines       12782    12783    +1     
=======================================
+ Hits        10014    10015    +1     
  Misses       2768     2768           

Flag	Coverage Δ
hatch-test.low-vers	77.64% <100.00%> (+<0.01%)	⬆️
hatch-test.pre	77.30% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	78.70% <100.00%> (+0.05%)	⬆️