[app-server] populate thread>turns>items on thread/resume (#6848)

This PR allows clients to render historical messages when resuming a
thread via `thread/resume` by reading from the list of `EventMsg`
payloads loaded from the rollout, and then transforming them into Turns
and ThreadItems to be returned on the `Thread` object.

This is implemented by leveraging `SessionConfiguredNotification` which
returns this list of `EventMsg` objects when resuming a conversation,
and then applying a stateful `ThreadHistoryBuilder` that parses from
this EventMsg log and transforms it into Turns and ThreadItems.

Note that we only persist a subset of `EventMsg`s in a rollout as
defined in `policy.rs`, so we lose fidelity whenever we resume a thread
compared to when we streamed the thread's turns originally. However,
this behavior is at parity with the legacy API.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -517,6 +517,10 @@ pub struct Thread {
     pub created_at: i64,
     /// [UNSTABLE] Path to the thread on disk.
     pub path: PathBuf,
+    /// Only populated on a `thread/resume` response.
+    /// For all other responses and notifications returning a Thread,
+    /// the turns field will be an empty list.
+    pub turns: Vec<Turn>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
@@ -531,8 +535,9 @@ pub struct AccountUpdatedNotification {
 #[ts(export_to = "v2/")]
 pub struct Turn {
     pub id: String,
-    /// This is currently only populated for resumed threads.
-    /// TODO: properly populate items for all turns.
+    /// Only populated on a `thread/resume` response.
+    /// For all other responses and notifications returning a Turn,
+    /// the items field will be an empty list.
     pub items: Vec<ThreadItem>,
     #[serde(flatten)]
     pub status: TurnStatus,