fix: ci's and scanpy 1.13 plot compat

[selmanozleyen]

This pull request improves compatibility with recent Scanpy releases, refactors how plotting palettes are imported, and reorganizes experimental plotting tests. It also adjusts test execution in pre-release environments to avoid failures due to external dependencies.

Scanpy compatibility and palette import refactor:

• Improved compatibility with Scanpy 1.13+ by updating imports in src/squidpy/_compat.py to use the new scanpy.plotting.legacy module when available, including fallback logic for older versions. The default_palette is now also imported via _compat.py and re-exported for use elsewhere.
• Updated src/squidpy/im/_container.py to import default_palette from squidpy._compat instead of directly from Scanpy, ensuring consistent palette usage regardless of Scanpy version.

Test organization and execution:

• Moved the visual regression test for stain normalization from tests/experimental/test_stain_normalize.py to a new file, tests/experimental/plotting/test_stain_normalize.py, and updated imports accordingly. [1] [2] [3] [4]
• Modified hatch.toml to skip running the experimental plotting tests in pre-release environments, preventing unrelated failures due to upstream changes in spatialdata-plot.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.21%. Comparing base (ca07e64) to head (f582a64).

Files with missing lines	Patch %	Lines
src/squidpy/_compat.py	70.83%	7 Missing ⚠️
src/squidpy/pl/_spatial_utils.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1253      +/-   ##
==========================================
- Coverage   77.24%   77.21%   -0.04%     
==========================================
  Files          63       63              
  Lines        9378     9396      +18     
  Branches     1579     1579              
==========================================
+ Hits         7244     7255      +11     
- Misses       1532     1539       +7     
  Partials      602      602              

Files with missing lines	Coverage Δ
src/squidpy/im/_container.py	85.40% <100.00%> (ø)
src/squidpy/pl/_spatial_utils.py	78.66% <66.66%> (-0.05%)	⬇️
src/squidpy/_compat.py	75.00% <70.83%> (-13.24%)	⬇️

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