Notify subscribers when thread read state changes (#52384)

## What changed

Add the experimental `thread/readState/changed` notification with `threadId` and the committed `readState` receipt. Send it to existing thread subscribers when terminal activity updates read state or a revert clears a removed unread position. Emit terminal read-state updates before `turn/completed` so completion remains the last notification for that result.

Expose the notification in generated JSON schemas, TypeScript types, and Python SDK models. Document experimental capability gating, subscription lifetime, notification opt-out, and refreshing receipts when concurrent responses disagree.

## Testing

Extend the read-state integration test to assert that the notification receipt matches the receipt returned by `thread/read` after terminal activity.

GitOrigin-RevId: 28935352a881b7a5e2e7f629173c3b2061bd7941

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

@@ -10249,6 +10249,14 @@ class ThreadReadState(BaseModel):
     revision: str
 
 
+class ThreadReadStateChangedNotification(BaseModel):
+    model_config = ConfigDict(
+        populate_by_name=True,
+    )
+    read_state: Annotated[ThreadReadState, Field(alias="readState")]
+    thread_id: Annotated[str, Field(alias="threadId")]
+
+
 class TranscriptSegmentThreadRealtimeItem(BaseModel):
     model_config = ConfigDict(
         populate_by_name=True,
@@ -11317,6 +11325,24 @@ class ErrorServerNotification(BaseModel):
     params: ErrorNotification
 
 
+class ThreadReadStateChangedServerNotification(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/readState/changed"],
+        Field(title="Thread/readState/changedNotificationMethod"),
+    ]
+    params: ThreadReadStateChangedNotification
+
+
 class ThreadGoalUpdatedServerNotification(BaseModel):
     model_config = ConfigDict(
         populate_by_name=True,
@@ -13116,6 +13142,7 @@ class ServerNotification(
         ErrorServerNotification
         | ThreadStartedServerNotification
         | ThreadStatusChangedServerNotification
+        | ThreadReadStateChangedServerNotification
         | ThreadArchivedServerNotification
         | ThreadDeletedServerNotification
         | ThreadUnarchivedServerNotification
@@ -13206,6 +13233,7 @@ class ServerNotification(
         ErrorServerNotification
         | ThreadStartedServerNotification
         | ThreadStatusChangedServerNotification
+        | ThreadReadStateChangedServerNotification
         | ThreadArchivedServerNotification
         | ThreadDeletedServerNotification
         | ThreadUnarchivedServerNotification