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

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -2303,12 +2303,24 @@ impl ThreadRequestProcessor {
                         codex_state::ReadStateOperation::Read,
                     )
                     .await
-                    .map(|_| ()),
-                Ok(None) => Ok(()),
+                    .map(|update| matches!(update, codex_state::ReadStateUpdate::Applied(_))),
+                Ok(None) => Ok(false),
                 Err(err) => Err(err),
             };
-            if let Err(err) = retirement {
-                tracing::warn!("reverted history but could not update unread position: {err}")
+            match retirement {
+                Ok(true) => {
+                    super::thread_read_state::notify(
+                        db,
+                        &self.thread_state_manager,
+                        &self.outgoing,
+                        thread_id,
+                    )
+                    .await;
+                }
+                Ok(false) => {}
+                Err(err) => {
+                    tracing::warn!("reverted history but could not update unread position: {err}")
+                }
             }
         }
         let response = self