Skip message-board notification previews when there are no recipients (#48072)

## What changed

Return post metadata from the in-memory message board after updating board state when no notification recipients remain. This avoids constructing an unused notification preview, including when the author is the only requested recipient.

## Testing

Run the existing post-and-search model-context scenario against both disk and in-memory message boards, checking that post metadata and search results reach the model without self-notifications.

GitOrigin-RevId: dcf07a5fd55738fbcd6edd3dce3f6fb8fc910eaf

codex-rs/core/tests/suite/scenarios_agent_message_board.rs

@@ -131,8 +131,12 @@ async fn board_requires_persistent_v2_runtime(
     Ok(())
 }
 
+#[test_case::test_case(false; "disk")]
+#[test_case::test_case(true; "in_memory")]
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn board_post_and_reads_reach_model_context_without_self_notices() -> anyhow::Result<()> {
+async fn board_post_and_reads_reach_model_context_without_self_notices(
+    in_memory: bool,
+) -> anyhow::Result<()> {
     let server = responses::start_mock_server().await;
     let mock = responses::mount_sse_sequence(&server, vec![
         tool("post-decision", "post", json!({"new_channel_name":"design", "text":"A shared decision.", "agents_to_notify":["/root"]})),
@@ -140,8 +144,9 @@ async fn board_post_and_reads_reach_model_context_without_self_notices() -> anyh
         done(),
     ]).await;
     let test = test_codex()
-        .with_config(|config| {
+        .with_config(move |config| {
             configure(config);
+            config.multi_agent_v2.message_board_in_memory = in_memory;
             config.current_time_reminder = Some(codex_core::config::CurrentTimeReminderConfig {
                 clock_source: codex_features::CurrentTimeSource::External,
                 ..Default::default()

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

@@ -326,6 +326,9 @@ impl InMemoryAgentMessageBoard {
                 }
             }
             state.subscribe(SubscriptionTarget::Thread(root), caller);
+            if recipients.is_empty() {
+                return Ok(metadata);
+            }
             (metadata, state.posts[index].preview(/*max_chars*/ 150))
         };
         futures::stream::iter(recipients).for_each_concurrent(/*limit*/ 16, |recipient| {