test(code-runtime-python): pin the pump reader against a class-attribute rebind; correct the staging comment

The review's three remaining items:
- A regression test rebinds __main__.ProtocolChannel.read_frame_async and asserts
  a binding reply still round-trips (the pump's reader is a bound method
  captured by _run before the program runs).
- The settlement note (en + zh) records that send_done's frame-shape check uses
  _run's bound _str/_isinstance.
- The staging-removal comment no longer claims teardown retries tracked state:
  teardown deliberately does not sweep staging, so a removal failure is the one
  case the gone-by-settlement contract degrades on.
Pairing re-recorded.

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

@@ -1704,9 +1704,11 @@ export class PythonCodeRuntime extends CodeRuntime {
           // Swallows only a failure to remove this run's staging directory —
           // `force` already absorbs a missing one, so what remains is a
           // filesystem-level refusal. The run's own outcome is already decided
-          // and must still be delivered, and teardown retries what stays
-          // tracked; the directory holds no secret, only a copy of two
-          // checked-in scripts.
+          // and must still be delivered; the directory holds no secret, only a
+          // copy of two checked-in scripts. teardown deliberately does not
+          // sweep staging (its staging is cleared inside each run's settle), so
+          // a removal failure here is the one case the "gone by settlement"
+          // contract degrades on.
         }
         resolve({ ...result, logs })
         // Mark the fiber quiescent for THIS run: drop it from `live` and resolve