[codex] Add docs regression checks

[iskandr]

Summary

• Add regression tests for the already-corrected documentation called out in Docs: run_isovar docstring has wrong parameter name and wrong return type #129, Docs: README example references non-existent property 'protein_sequence_matches_predicted_effect' #133, and Docs: DataFrameBuilder 'exclude' docstring says the opposite of what the code does #135.
• Verify the run_isovar docstring names decompression_threads and documents a list return type.
• Verify the README filter flag example uses protein_sequence_matches_predicted_mutation_effect.
• Verify DataFrameBuilder.exclude is documented as omitting fields.
• Bump isovar to 1.7.1 per repository release policy.

Closes #129.
Closes #133.
Closes #135.

Validation

• pytest tests/test_docs.py tests/test_main.py::test_run_isovar_docstring_uses_current_filter_api_names tests/test_dataframe_builder.py::test_dataframe_rename_and_converters_and_exclude
• ./lint.sh
• ./test.sh - 217 passed

[coveralls]

coverage: 93.768%. remained the same — codex/docs-cleanup-129-133-135 into master