Fix #4008: Opinionated defaults for 2.0

[JiwaniZakir]

Closes #4008

• Closes Opinionated defaults for 2.0 #4008
• Tests included or not required because: added test_v2preview_preset_mask_var in tests/test_rank_genes_groups.py
• Release notes not necessary because:

Sets mask_var="highly_variable" as the default for RankGenesGroupsPreset under the ScanpyV2Preview preset in src/scanpy/_settings/presets.py. Previously both ScanpyV1 and ScanpyV2Preview used mask_var=None; now ScanpyV2Preview automatically restricts differential expression to highly variable genes by default, aligning with the opinionated v2.0 conventions described in #4008.

The new unit test test_v2preview_preset_mask_var directly asserts:

• Preset.ScanpyV2Preview.rank_genes_groups.mask_var == "highly_variable"
• Preset.ScanpyV1.rank_genes_groups.mask_var is None

---

This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.61%. Comparing base (6e7232c) to head (529b780).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4063      +/-   ##
==========================================
- Coverage   78.62%   78.61%   -0.01%     
==========================================
  Files         117      117              
  Lines       12727    12713      -14     
==========================================
- Hits        10006     9994      -12     
+ Misses       2721     2719       -2     

Flag	Coverage Δ
hatch-test.low-vers	77.91% <ø> (-0.01%)	⬇️
hatch-test.pre	78.49% <ø> (-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/_settings/presets.py	91.08% <ø> (ø)

... and 4 files with indirect coverage changes

[flying-sheep]

Hi, that issue isn’t actionable yet, we’re still deciding what the defaults are going to be