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 81.88%. Comparing base (ec37402) to head (d1c23f2).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4325      +/-   ##
==========================================
- Coverage   81.98%   81.88%   -0.11%     
==========================================
  Files         134      134              
  Lines       13235    13254      +19     
==========================================
+ Hits        10851    10853       +2     
- Misses       2384     2401      +17     

Flag	Coverage Δ
hatch-test.low-vers	?
hatch-test.pre	81.88% <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%)	⬆️

... and 5 files with indirect coverage changes

[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.

[AtomicGlance]

CI reran on the latest commit. The PAGA-focused tests pass and Codecov reports 100% patch coverage.\n\nThe remaining Python 3.14 failure is unrelated to this change: ests/test_logging.py::test_logfile finishes with 3,033 tests passed and one teardown error from an unclosed-file ResourceWarning. That test is unchanged in this PR and the diff is limited to PAGA code, tests, and the release-note fragment, so I have not added an unrelated fix.\n\nThe title/milestone/labels check is also still blocked because this fork cannot add the repository's no-milestone label or assign a milestone. Could a maintainer handle that metadata check and advise whether the Python 3.14 logging failure should be rerun or tracked upstream?