fix(code-runtime-python): close the child stdin handle and def-time capture the frame decode primitives

Addresses the review's two remaining items:
- The host closes the child's stdin write handle immediately after spawn. The
  program is an async body that reads nothing from fd 0; a live pipe would hold
  a host-side handle open past the run, so a setsid-escaped descendant
  inheriting fd 0 could keep the host process from exiting even after the
  closeDeadline forced settlement. The child (and any descendant) reads EOF on
  fd 0 and no host handle survives.
- read_frame/read_frame_async bind their decode primitives (_decode_json_plain,
  os.read, _READ_CHUNK_BYTES, bytes) as def-time default arguments, and
  _decode_json_plain itself captures json.loads, its two regexes, and len the
  same way, so a __main__ rebind cannot kill the reply pump and strand every
  pending Future to the wall clock. _decode_json_plain and its regexes moved
  before the ProtocolChannel class so the defaults resolve at class-definition
  time. A regression test rebinds _decode_json_plain and asserts a binding reply
  still round-trips.
Note (en + zh) registers both mechanisms; pairings re-recorded.

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

@@ -989,6 +989,13 @@ export class PythonCodeRuntime extends CodeRuntime {
       if (proto === null) {
         throw new Error('dsh-code-runtime-python: python subprocess spawned without a fd-3 pipe')
       }
+      // Close the host's stdin write handle immediately: the program is an
+      // async body that reads nothing from fd 0, and a live pipe here would
+      // hold a host-side handle open past the run — a setsid-escaped descendant
+      // inheriting fd 0 would keep the host process from exiting even after the
+      // closeDeadline forced settlement. The child (and any descendant) reads
+      // EOF on fd 0 instead, and no host handle survives.
+      child.stdin.destroy()
     } catch (error: unknown) {
       try {
         rmSync(bootstrapDir, { recursive: true, force: true })