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/generated/notification_registry.py

@@ -130,3 +130,43 @@ NOTIFICATION_MODELS: dict[str, type[BaseModel]] = {
     "windows/worldWritableWarning": WindowsWorldWritableWarningNotification,
     "windowsSandbox/setupCompleted": WindowsSandboxSetupCompletedNotification,
 }
+
+DIRECT_TURN_ID_NOTIFICATION_TYPES: tuple[type[BaseModel], ...] = (
+    AgentMessageDeltaNotification,
+    CommandExecutionOutputDeltaNotification,
+    ContextCompactedNotification,
+    ErrorNotification,
+    FileChangeOutputDeltaNotification,
+    FileChangePatchUpdatedNotification,
+    HookCompletedNotification,
+    HookStartedNotification,
+    ItemCompletedNotification,
+    ItemGuardianApprovalReviewCompletedNotification,
+    ItemGuardianApprovalReviewStartedNotification,
+    ItemStartedNotification,
+    McpToolCallProgressNotification,
+    ModelReroutedNotification,
+    ModelVerificationNotification,
+    PlanDeltaNotification,
+    ReasoningSummaryPartAddedNotification,
+    ReasoningSummaryTextDeltaNotification,
+    ReasoningTextDeltaNotification,
+    TerminalInteractionNotification,
+    ThreadGoalUpdatedNotification,
+    ThreadTokenUsageUpdatedNotification,
+    TurnDiffUpdatedNotification,
+    TurnPlanUpdatedNotification,
+)
+
+NESTED_TURN_NOTIFICATION_TYPES: tuple[type[BaseModel], ...] = (
+    TurnCompletedNotification,
+    TurnStartedNotification,
+)
+
+
+def notification_turn_id(payload: BaseModel) -> str | None:
+    if isinstance(payload, DIRECT_TURN_ID_NOTIFICATION_TYPES):
+        return payload.turn_id if isinstance(payload.turn_id, str) else None
+    if isinstance(payload, NESTED_TURN_NOTIFICATION_TYPES):
+        return payload.turn.id
+    return None