fix(test): isolate interpreter-discovery tests from ambient SYSTEM_PYTHON

The compat job exports SYSTEM_PYTHON for the venv-building CLI tests;
the discovery unit tests fabricate their own interpreters and the
ambient override short-circuited the walk they exercise. An autouse
fixture clears it — verified passing both with and without the
variable set.

Author: rahlk

test/test_env_interpreter.py

@@ -20,6 +20,14 @@
 # ----------------------------------------------------------------------------------------------
 
 
+@pytest.fixture(autouse=True)
+def _no_ambient_system_python(monkeypatch):
+    """Discovery tests fabricate their own interpreters; an ambient
+    SYSTEM_PYTHON (the CI compat job sets one for the venv-building CLI
+    tests) would short-circuit the walk they are exercising."""
+    monkeypatch.delenv("SYSTEM_PYTHON", raising=False)
+
+
 def test_grammar_stems_parse_to_versions():
     got = Codeanalyzer._versions_from_grammar_stems(
         ["grammar36", "grammar39", "grammar310", "grammar313", "grammar314"]