Broadcast thread section changes to connected clients (#58380)

## Why

Clients need to refresh section membership and ordering when another client moves a thread.

## What changed

Broadcast `thread/section/changed` after a successful `thread/section/move`, including moves within the same section. Send the response before the notification, and include `threadId` and the destination `sectionId`, using `null` for removal. Failed moves do not emit the notification.

Add the notification to the protocol schemas, TypeScript exports, and Python SDK models and registry, and document its behavior.

## Testing

Add a WebSocket integration test with separate writer and observer clients covering section insertion, reordering, removal, and a failed move.

GitOrigin-RevId: 128ba7eddc9aae8e3bac766f47e4a2c55c64a6a5

sdk/python/src/openai_codex/generated/v2_all.py

@@ -6308,6 +6308,20 @@ class ThreadSectionAppearance(BaseModel):
     icon: str | None = None
 
 
+class ThreadSectionChangedNotification(BaseModel):
+    model_config = ConfigDict(
+        populate_by_name=True,
+    )
+    section_id: Annotated[
+        str | None,
+        Field(
+            alias="sectionId",
+            description="Destination section, or `null` when the thread was removed from its section.",
+        ),
+    ]
+    thread_id: Annotated[str, Field(alias="threadId")]
+
+
 class ThreadSectionCreateParams(BaseModel):
     model_config = ConfigDict(
         populate_by_name=True,
@@ -9697,6 +9711,23 @@ class ThreadNameUpdatedServerNotification(BaseModel):
     params: ThreadNameUpdatedNotification
 
 
+class ThreadSectionChangedServerNotification(BaseModel):
+    model_config = ConfigDict(
+        populate_by_name=True,
+    )
+    emitted_at_ms: Annotated[
+        int | None,
+        Field(
+            alias="emittedAtMs",
+            description="Unix timestamp (in milliseconds) when app-server emitted this notification.",
+        ),
+    ] = None
+    method: Annotated[
+        Literal["thread/section/changed"], Field(title="Thread/section/changedNotificationMethod")
+    ]
+    params: ThreadSectionChangedNotification
+
+
 class ThreadAttachmentUpdatedServerNotification(BaseModel):
     model_config = ConfigDict(
         populate_by_name=True,
@@ -13634,6 +13665,7 @@ class ServerNotification(
         | ThreadRevertedServerNotification
         | SkillsChangedServerNotification
         | ThreadNameUpdatedServerNotification
+        | ThreadSectionChangedServerNotification
         | ThreadAttachmentUpdatedServerNotification
         | ThreadGoalUpdatedServerNotification
         | ThreadPredictionUpdatedServerNotification
@@ -13722,6 +13754,7 @@ class ServerNotification(
         | ThreadRevertedServerNotification
         | SkillsChangedServerNotification
         | ThreadNameUpdatedServerNotification
+        | ThreadSectionChangedServerNotification
         | ThreadAttachmentUpdatedServerNotification
         | ThreadGoalUpdatedServerNotification
         | ThreadPredictionUpdatedServerNotification