fix(runtime): enforce functional containment after closing execution bypasses (ODY-141)

tests/containment_helpers.py

@@ -14,18 +14,26 @@ def capture_owned_spawn(monkeypatch, tmp_path):
     async def fake_exec(*argv, **kwargs):
         captured.update(argv=argv, kwargs=kwargs, command=argv[-1])
         stdout = asyncio.StreamReader()
+        if "ody-boundary" in argv:
+            stdout.feed_data((argv[argv.index("ody-boundary") + 1] + "\n").encode())
         stdout.feed_data(b"ok")
         stdout.feed_eof()
         stderr = asyncio.StreamReader()
         stderr.feed_eof()
         async def wait():
             return 0
+        async def drain():
+            return None
+        writer = SimpleNamespace(write=lambda data: None, drain=drain,
+                                 close=lambda: captured.update(stdin_closed=True))
         return SimpleNamespace(pid=99999999, stdout=stdout, stderr=stderr,
-                               returncode=0, wait=wait)
+                               stdin=writer, returncode=0, wait=wait)
 
     async def release(*args, **kwargs):
         return containment.ReleaseOutcome(dead=True, escalated=False)
 
     monkeypatch.setattr(asyncio, "create_subprocess_exec", fake_exec)
+    if hasattr(containment.os, "pidfd_open"):
+        monkeypatch.delattr(containment.os, "pidfd_open")
     monkeypatch.setattr(containment, "_release_awaited", release)
     return captured