Improved error message for incorrect gene_symbols columns

[SAY-5]

Closes #618.

_validate_col_for_column_table swallowed the KeyError from _resolve_gene_symbols on the auto-detect branch, so a wrong gene_symbols= column name surfaced as the generic "Unable to locate color key" message. Now the lookup column is validated against every annotating table's var columns before the per-table loop, so a typo raises immediately with the available column names.

Added a regression test on the auto-detect path (without table_name=).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.49%. Comparing base (84e5da7) to head (3cdd83f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #658      +/-   ##
==========================================
+ Coverage   77.07%   77.49%   +0.41%     
==========================================
  Files          11       11              
  Lines        3280     3572     +292     
  Branches      774      838      +64     
==========================================
+ Hits         2528     2768     +240     
- Misses        454      481      +27     
- Partials      298      323      +25     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	67.62% <100.00%> (+0.29%)	⬆️

... and 3 files with indirect coverage changes

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[timtreis]

Hey @SAY-5, thank you for your contribution.