feat: introduce GetConversationSummary RPC (#5803)

This adds an RPC to the app server to the the `ConversationSummary` via
a rollout path. Now that the VS Code extension supports showing the
Codex UI in an editor panel where the URI of the panel maps to the
rollout file, we need to be able to get the `ConversationSummary` from
the rollout file directly.

codex-rs/core/src/lib.rs

@@ -77,6 +77,7 @@ pub use rollout::find_conversation_path_by_id_str;
 pub use rollout::list::ConversationItem;
 pub use rollout::list::ConversationsPage;
 pub use rollout::list::Cursor;
+pub use rollout::list::read_head_for_summary;
 mod function_tool;
 mod state;
 mod tasks;

codex-rs/core/src/rollout/list.rs

@@ -451,6 +451,13 @@ async fn read_head_and_tail(
     Ok(summary)
 }
 
+/// Read up to `HEAD_RECORD_LIMIT` records from the start of the rollout file at `path`.
+/// This should be enough to produce a summary including the session meta line.
+pub async fn read_head_for_summary(path: &Path) -> io::Result<Vec<serde_json::Value>> {
+    let summary = read_head_and_tail(path, HEAD_RECORD_LIMIT, 0).await?;
+    Ok(summary.head)
+}
+
 async fn read_tail_records(
     path: &Path,
     max_records: usize,