feat: add configurable columns to PAGA plots

[AtomicGlance]

Summary

• Add an ncols parameter to sc.pl.paga for wrapping multiple color panels into a grid.
• Preserve the existing single-row layout when ncols=None.
• Keep colorbars aligned with grid panels and validate invalid column counts.
• Add regression coverage and a release-note fragment.

Closes #1203

Tests

• pytest -q tests/plotting/legacy/test_paga.py -k paga_ncols -o filterwarnings=: 8 passed
• Ruff, Python byte-compilation, and git diff --check pass.
• The rest of test_paga.py passes 24 tests; one unrelated test needs to download the existing pbmc3k fixture, which is blocked in this environment.

[AtomicGlance]

The code and focused tests are in place. One repository metadata check is failing only because this fork PR cannot add the required no milestone label without upstream permissions. Could a maintainer add that label or assign a milestone?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.17%. Comparing base (84b6b1b) to head (b6c4457).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4325      +/-   ##
==========================================
+ Coverage   80.14%   80.17%   +0.02%     
==========================================
  Files         133      133              
  Lines       13478    13497      +19     
==========================================
+ Hits        10802    10821      +19     
  Misses       2676     2676              

Flag	Coverage Δ
hatch-test.low-vers	77.87% <42.85%> (-0.06%)	⬇️
hatch-test.pre	80.04% <100.00%> (+0.02%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/plotting/legacy/_tools/paga.py	65.53% <100.00%> (+1.14%)	⬆️

[AtomicGlance]

Thanks for flagging this. I added coverage for invalid ncols values, the ncols plus pre-supplied ax error, and a custom colorbar axis, then pushed b6c4457. The focused PAGA tests now pass 11 cases; Codecov should rerun on the new commit.