test(code-runtime-python): cover the reply-pump closed-loop guard; align setsid docs

The closed-loop reply-pump guard now ships with a deterministic regression test:
a worker thread abandons a binding so its loop closes, the host answers that call
before a later binding, and the pump must survive the closed-loop
call_soon_threadsafe to deliver the later reply (host-gated ordering makes it
deterministic; unguarding the pump hangs the later binding to the wall clock).

Align the quiescence self-description with the shipped setsid limitation:
teardown()'s JSDoc and the Agent Note's Problem line now qualify "no subprocess
outlives the fiber" to subprocesses that stay in the child's process group, with
a setsid()-escape exception pointing at the README. Tighten the setsid-orphan
fixture's self-timeout to 5s and its upper-bound assertion to <4000ms so a failed
deadline backstop is a sharper red. Register the new regression tests in the note.

packages/code-runtime/code-runtime-python/src/index.ts

@@ -527,7 +527,11 @@ export class PythonCodeRuntime extends CodeRuntime {
 
   /**
    * Dispose to quiescence: fail every in-flight run as aborted and AWAIT each
-   * child's exit so no subprocess outlives the fiber.
+   * child's exit so no subprocess that stays in the child's process group
+   * outlives the fiber. A descendant that escaped the group with `setsid()` /
+   * `start_new_session=True` is unreachable by `kill(-pid)` and is the documented
+   * exception (see the package README's Known Limitations); the process-group
+   * teardown reaps everything that stays in the group.
    */
   private async teardown(): Promise<void> {
     this.disposed = true