Route Python SDK turn notifications by ID (#21778)

## Why

The Python SDK previously protected the stdio transport with a single
active turn-consumer guard. That avoided competing reads from stdout,
but it also meant one `Codex`/`AsyncCodex` client could not stream
multiple active turns at the same time. Notifications could also arrive
before the caller received a `TurnHandle` and registered for streaming,
so the SDK needed an explicit routing layer instead of letting
individual API calls read directly from the shared transport.

## What Changed

- Added a private `MessageRouter` that owns per-request response queues,
per-turn notification queues, pending turn-notification replay, and
global notification delivery behind a single stdout reader thread.
- Generated typed notification routing metadata so turn IDs come from
known payload shapes instead of router-side attribute guessing, with
explicit fallback handling for unknown notification payloads.
- Updated sync and async turn streaming so `TurnHandle.stream()`/`run()`
and `stream_text()` consume only notifications for their own turn ID,
while `AsyncAppServerClient` no longer serializes all transport calls
behind one async lock.
- Cleared pending turn-notification buffers when unregistered turns
complete so never-consumed turn handles do not leave stale queues
behind.
- Removed the internal stream-until helper now that turn completion
waiting can register directly with routed turn notifications.
- Updated Python SDK docs and focused tests for concurrent transport
calls, interleaved turn routing, buffered early notifications, unknown
notification routing, async delegation, and routed turn completion
behavior.

## Validation

- `uv run --extra dev ruff format scripts/update_sdk_artifacts.py
src/codex_app_server/_message_router.py src/codex_app_server/client.py
src/codex_app_server/generated/notification_registry.py
tests/test_client_rpc_methods.py
tests/test_public_api_runtime_behavior.py
tests/test_async_client_behavior.py`
- `uv run --extra dev ruff check scripts/update_sdk_artifacts.py
src/codex_app_server/_message_router.py src/codex_app_server/client.py
src/codex_app_server/generated/notification_registry.py
tests/test_client_rpc_methods.py
tests/test_public_api_runtime_behavior.py
tests/test_async_client_behavior.py`
- `uv run --extra dev pytest tests/test_client_rpc_methods.py
tests/test_public_api_runtime_behavior.py
tests/test_async_client_behavior.py`
- `git diff --check`

---------

Co-authored-by: Codex <noreply@openai.com>

sdk/python/src/codex_app_server/api.py

@@ -38,14 +38,14 @@ from .generated.v2_all import (
 )
 from .models import InitializeResponse, JsonObject, Notification, ServerInfo
 from ._inputs import (
-    ImageInput,
+    ImageInput as ImageInput,
     Input,
-    InputItem,
-    LocalImageInput,
-    MentionInput,
+    InputItem as InputItem,
+    LocalImageInput as LocalImageInput,
+    MentionInput as MentionInput,
     RunInput,
-    SkillInput,
-    TextInput,
+    SkillInput as SkillInput,
+    TextInput as TextInput,
     _normalize_run_input,
     _to_wire_input,
 )
@@ -274,6 +274,7 @@ class Codex:
     def thread_unarchive(self, thread_id: str) -> Thread:
         unarchived = self._client.thread_unarchive(thread_id)
         return Thread(self._client, unarchived.thread.id)
+
     # END GENERATED: Codex.flat_methods
 
     def models(self, *, include_hidden: bool = False) -> ModelListResponse:
@@ -476,6 +477,7 @@ class AsyncCodex:
         await self._ensure_initialized()
         unarchived = await self._client.thread_unarchive(thread_id)
         return AsyncThread(self, unarchived.thread.id)
+
     # END GENERATED: AsyncCodex.flat_methods
 
     async def models(self, *, include_hidden: bool = False) -> ModelListResponse:
@@ -555,6 +557,7 @@ class Thread:
         )
         turn = self._client.turn_start(self.id, wire_input, params=params)
         return TurnHandle(self._client, self.id, turn.turn.id)
+
     # END GENERATED: Thread.flat_methods
 
     def read(self, *, include_turns: bool = False) -> ThreadReadResponse:
@@ -644,6 +647,7 @@ class AsyncThread:
             params=params,
         )
         return AsyncTurnHandle(self._codex, self.id, turn.turn.id)
+
     # END GENERATED: AsyncThread.flat_methods
 
     async def read(self, *, include_turns: bool = False) -> ThreadReadResponse:
@@ -674,11 +678,10 @@ class TurnHandle:
         return self._client.turn_interrupt(self.thread_id, self.id)
 
     def stream(self) -> Iterator[Notification]:
-        # TODO: replace this client-wide experimental guard with per-turn event demux.
-        self._client.acquire_turn_consumer(self.id)
+        self._client.register_turn_notifications(self.id)
         try:
             while True:
-                event = self._client.next_notification()
+                event = self._client.next_turn_notification(self.id)
                 yield event
                 if (
                     event.method == "turn/completed"
@@ -687,7 +690,7 @@ class TurnHandle:
                 ):
                     break
         finally:
-            self._client.release_turn_consumer(self.id)
+            self._client.unregister_turn_notifications(self.id)
 
     def run(self) -> AppServerTurn:
         completed: TurnCompletedNotification | None = None
@@ -728,11 +731,10 @@ class AsyncTurnHandle:
 
     async def stream(self) -> AsyncIterator[Notification]:
         await self._codex._ensure_initialized()
-        # TODO: replace this client-wide experimental guard with per-turn event demux.
-        self._codex._client.acquire_turn_consumer(self.id)
+        self._codex._client.register_turn_notifications(self.id)
         try:
             while True:
-                event = await self._codex._client.next_notification()
+                event = await self._codex._client.next_turn_notification(self.id)
                 yield event
                 if (
                     event.method == "turn/completed"
@@ -741,7 +743,7 @@ class AsyncTurnHandle:
                 ):
                     break
         finally:
-            self._codex._client.release_turn_consumer(self.id)
+            self._codex._client.unregister_turn_notifications(self.id)
 
     async def run(self) -> AppServerTurn:
         completed: TurnCompletedNotification | None = None