test: codify process-state isolation rules

Author: morluto

docs/reference/testing-strategy.md

@@ -686,6 +686,22 @@ The checked-in JSON Schemas should be exercised both through Pydantic and a
 standards-oriented JSON Schema validator. A Pydantic model successfully reading
 its own generated schema is not independent contract evidence.
 
+### Pytest antipatterns
+
+- Use `monkeypatch` for process-global state such as `sys.modules`, `sys.argv`,
+  and environment variables. When restoration is the behavior under test,
+  assert that the original object or missing state is restored.
+- Name the expected exception type and a stable diagnostic instead of using
+  bare `pytest.raises(Exception)`.
+- Assert the observable outcome or independently parsed wire contract; a model
+  successfully validating its own output is not sufficient evidence.
+- Name source-shape checks honestly and reserve them for supported text or
+  architecture contracts. Do not disguise source substrings as behavior tests.
+- Do not add an empty `pytestmark`; markers exist only for execution-affecting
+  traits owned by the test topology.
+- Keep Harbor regressions in their owning dataset and task leaf. Shared
+  validation modules should contain only suite-wide contracts.
+
 Use the standard library's `tempfile`, `subprocess`, and process primitives for
 artifact and replay tests. Do not use an in-memory filesystem or `pyfakefs` to
 claim evidence about atomic rename, SQLite WAL recovery, symlink handling, or

tests/unit/tooling/test_ci_planner_catalog.py

@@ -36,13 +36,10 @@ def _load_script(name: str) -> ModuleType:
 def _ci_plan(*args: str) -> dict[str, str]:
     classifier = _load_script("classify-ci-paths")
     output = io.StringIO()
-    original_argv = sys.argv
-    try:
-        sys.argv = ["classify-ci-paths", *args]
+    with pytest.MonkeyPatch.context() as process_state:
+        process_state.setattr(sys, "argv", ["classify-ci-paths", *args])
         with contextlib.redirect_stdout(output):
             classifier.main()
-    finally:
-        sys.argv = original_argv
     return dict(line.split("=", 1) for line in output.getvalue().splitlines())