Stale conftest docstring references retired codeanalyzer-native binary

[rahlk]

The codeanalyzer_backend_path fixture in tests/conftest.py still documents the retired native binary:

Returns None so the analyzer uses its default: the JVM-free native binary shipped in the codeanalyzer-java PyPI package (python -m codeanalyzer_java).

There is no codeanalyzer-native build anymore (see #290). The analyzer's default is now the bundled codeanalyzer-*.jar run on a cached JDK ([java, -jar, <jar>], via _get_codeanalyzer_exec()). Update the docstring to match; the fixture still correctly returns None.