test: address review feedback on fake_process fixture

- document str defaults / bytes for binary-mode callers
- wire __exit__ to False so exceptions are not suppressed
- exercise exited-process path through subprocess.run

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/conftest.py

@@ -147,6 +147,8 @@ def fake_process():
     Hand-rolled Popen mocks keep missing two things: __enter__ (subprocess.run
     wraps Popen in a context manager) and a communicate() tuple. Use this
     factory instead of building MagicMock processes inline.
+
+    Defaults are str (for text=True subprocesses); pass bytes explicitly for binary-mode callers.
     """
 
     def _make(returncode=0, stdout="", stderr="", running=True, pid=12345):
@@ -160,6 +162,7 @@ def fake_process():
         proc.stderr.read.return_value = stderr
         proc.stdin = MagicMock()
         proc.__enter__.return_value = proc
+        proc.__exit__.return_value = False
         return proc
 
     return _make

tests/test_conftest_fixtures.py

@@ -18,7 +18,10 @@ class TestFakeProcess:
         assert proc.pid == 12345
         assert proc.communicate() == ("", "")
 
-    def test_exited_process(self, fake_process):
-        proc = fake_process(running=False, returncode=1, stderr=b"device busy")
-        assert proc.poll() == 1
-        assert proc.stderr.read() == b"device busy"
+    def test_exited_process_detected_via_run(self, fake_process):
+        """An exited process's returncode and stderr surface through subprocess.run."""
+        proc = fake_process(running=False, returncode=1, stdout=b"", stderr=b"device busy")
+        with patch("subprocess.Popen", return_value=proc):
+            result = subprocess.run(["anything"], capture_output=True)
+        assert result.returncode == 1
+        assert result.stderr == b"device busy"