fix(config): honor CORS_* environment variables

[alukach]

The documented CORS_ALLOW_ORIGINS, CORS_ALLOW_METHODS, etc. environment variables were silently ignored: with env_nested_delimiter="_", pydantic-settings exploded CORS_ALLOW_ORIGINS into cors.allow.origins, which was dropped. Only the CORS={...} JSON form worked, so restricted origins silently stayed *. This happens with every pydantic-settings version I tried (2.10.1 through 2.14.1), not only the latest.

Changes:

• Set env_nested_max_split=1 so CORS_ALLOW_ORIGINS maps to cors.allow_origins. ITEMS_FILTER_CLS/ARGS/KWARGS are unaffected.
• Mark the CORS list fields NoDecode so the documented comma-separated form works, the same way root_path_skip_prefixes already does.
• Raise the minimum to pydantic-settings>=2.8.0; 2.7.0 doesn't support env_nested_max_split, which I confirmed by testing it.

Tests cover the CORS_* form, the CORS={...} JSON form, and ITEMS_FILTER_* still parsing. test_cors_env_vars fails on main.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (be008ba) to head (018cbb4).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   89.76%   89.74%   -0.02%     
==========================================
  Files          30       30              
  Lines        1348     1346       -2     
  Branches      180      179       -1     
==========================================
- Hits         1210     1208       -2     
  Misses         97       97              
  Partials       41       41              

Flag	Coverage Δ
unittests	89.74% <100.00%> (-0.02%)	⬇️

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