Prune expired in-memory message board registry entries (#47952)

## Why

Dropping the last board handle releases its state but leaves a stale entry in
`InMemoryMessageBoards`, allowing registry entries to accumulate across sessions.

## What changed

Prune entries whose state has no remaining strong references whenever a board is
opened, while preserving shared state for live boards.

## Testing

Add a regression test that repeatedly opens and drops boards, checks that expired
entries are removed, and verifies that reopening a live board shares its state.

GitOrigin-RevId: c3f1c27b42ee17915f0ece01b85a22d0fdf632ce

codex-rs/ext/agent-message-board/src/in_memory.rs

@@ -1,5 +1,6 @@
 //! Tree-owned message boards for training. Agent handles share state; nothing is written to disk.
 //! Mutations and recipient selection are atomic. Host callbacks run outside the state lock.
+//! Opening a board prunes registry entries whose state has been released.
 
 use crate::ChannelSummary;
 use crate::CreateChannelRequest;
@@ -31,6 +32,10 @@ use uuid::Uuid;
 
 mod queries;
 
+#[cfg(test)]
+#[path = "in_memory_tests.rs"]
+mod tests;
+
 /// Shares board state across a tree while giving each agent its own host handle.
 #[derive(Default)]
 pub struct InMemoryMessageBoards {
@@ -44,6 +49,7 @@ impl InMemoryMessageBoards {
         host: Arc<dyn MessageBoardHost>,
     ) -> InMemoryAgentMessageBoard {
         let mut states = self.states.lock().await;
+        states.retain(|_, state| state.strong_count() > 0);
         let entry = states.entry(identity).or_default();
         let state = entry.upgrade().unwrap_or_else(|| {
             let state = Arc::default();

codex-rs/ext/agent-message-board/src/in_memory_tests.rs

@@ -0,0 +1,66 @@
+//! Verifies that registry cleanup releases expired entries and preserves live boards.
+
+use super::*;
+use crate::NotificationDelivery;
+use futures::future::BoxFuture;
+use pretty_assertions::assert_eq;
+
+struct UnusedHost;
+
+impl MessageBoardHost for UnusedHost {
+    fn agent_path(&self, _caller: ThreadId) -> BoxFuture<'_, Result<AgentPath>> {
+        unreachable!("registry tests do not invoke host capabilities")
+    }
+
+    fn resolve_agent(&self, _path: AgentPath) -> BoxFuture<'_, Result<ThreadId>> {
+        unreachable!("registry tests do not invoke host capabilities")
+    }
+
+    fn current_time(&self, _caller: ThreadId) -> BoxFuture<'_, Result<DateTime<Utc>>> {
+        unreachable!("registry tests do not invoke host capabilities")
+    }
+
+    fn notify(
+        &self,
+        _recipient: ThreadId,
+        _post: PostPreview,
+    ) -> BoxFuture<'_, Result<NotificationDelivery>> {
+        unreachable!("registry tests do not invoke host capabilities")
+    }
+}
+
+#[tokio::test]
+async fn opening_boards_prunes_expired_entries_and_preserves_live_state() {
+    let boards = InMemoryMessageBoards::default();
+    let identity = SessionId::new();
+    let live = boards.open(identity, Arc::new(UnusedHost)).await;
+
+    for _ in 0..16 {
+        let next_identity = SessionId::new();
+        let next = boards.open(next_identity, Arc::new(UnusedHost)).await;
+        assert_eq!(
+            boards
+                .states
+                .lock()
+                .await
+                .keys()
+                .copied()
+                .collect::<HashSet<_>>(),
+            HashSet::from([identity, next_identity]),
+        );
+        drop(next);
+    }
+
+    let shared = boards.open(identity, Arc::new(UnusedHost)).await;
+    assert!(Arc::ptr_eq(&live.state, &shared.state));
+    assert_eq!(
+        boards
+            .states
+            .lock()
+            .await
+            .keys()
+            .copied()
+            .collect::<HashSet<_>>(),
+        HashSet::from([identity]),
+    );
+}