fix: guard section params before validator setup

[gaoflow]

Summary

• raise BSBLANError(API_VALIDATOR_NOT_INITIALIZED) when SectionValidator.get_section_params() is called before setup
• add regression coverage for the uninitialized lookup path

Fixes #1521.

Tests

• uv run pytest --no-cov tests/test_api_validation.py -k 'get_section_params_no_validator or validate_api_section_no_validator'\n- uv run pytest --no-cov tests/test_api_validation.py\n- uv run prek run --all-files

[gaoflow]

Correction: the missing required label is bugfix. I do not have permission to add labels from the fork; could a maintainer please add it?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (59b5311) to head (5cbbd88).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1544   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1233      1235    +2     
  Branches       141       142    +1     
=========================================
+ Hits          1233      1235    +2     

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[liudger]

LGTM thanks for the pull request