Add async delivery metadata to agent messages (#39312)

## What changed

- Add an optional `delivery` field to agent message events and app-server
  `agentMessage` items, with `"async"` identifying a user-visible message sent
  without ending the current turn.
- Preserve the marker through legacy event conversion, thread history
  materialization, replay, and generated JSON and TypeScript schemas.
- Require delivery of async agent message completion notifications on the
  in-process app-server transport.

## Testing

- Cover preservation of async delivery metadata in thread history.
- Cover delivery-required classification for async agent messages.

GitOrigin-RevId: f92ba25de1293ee271404badfdf0d117d6530329

codex-rs/protocol/src/items.rs

@@ -113,6 +113,14 @@ pub enum AgentMessageContent {
     Text { text: String },
 }
 
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, TS, JsonSchema, PartialEq, Eq)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum AgentMessageDelivery {
+    Async,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
 /// Assistant-authored message payload used in turn-item streams.
 ///
@@ -132,6 +140,9 @@ pub struct AgentMessageItem {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]
     pub memory_citation: Option<MemoryCitation>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub delivery: Option<AgentMessageDelivery>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]

codex-rs/protocol/src/legacy_events.rs

@@ -105,6 +105,7 @@ impl AgentMessageItem {
                     message: text.clone(),
                     phase: self.phase.clone(),
                     memory_citation: self.memory_citation.clone(),
+                    delivery: self.delivery,
                 }),
             })
             .collect()

codex-rs/protocol/src/protocol.rs

@@ -32,6 +32,7 @@ use crate::dynamic_tools::DynamicToolCallRequest;
 use crate::dynamic_tools::DynamicToolResponse;
 use crate::dynamic_tools::DynamicToolSpec;
 use crate::error::Result as CodexResult;
+use crate::items::AgentMessageDelivery;
 use crate::items::TurnItem;
 use crate::mcp::CallToolResult;
 use crate::mcp::RequestId;
@@ -2336,6 +2337,9 @@ pub struct AgentMessageEvent {
     pub phase: Option<MessagePhase>,
     #[serde(default)]
     pub memory_citation: Option<MemoryCitation>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub delivery: Option<AgentMessageDelivery>,
 }
 
 #[derive(Debug, Clone, Default, Deserialize, Serialize, PartialEq, JsonSchema, TS)]