Session picker shows thread_name if set (#10340)

- shows names of threads in the ResumePicker used by `/resume` and
`codex resume` if set, default to preview (previous behaviour) if none
- adds a `find_thread_names_by_ids` that maps names to IDs in
`codex-rs/core/src/rollout/session_index.rs`. It reads sequentially in
normal (instead of reverse order in `codex resume <name>`) the index
mapping file. This function is called from a list of session (default
page is 25, pages loaded depends of height of terminal), for which most
of them will always have at least one session unnamed and require the
whole file to be read therefore. Could be better and sqlite integration
will make this better
- those reads won't be needed when leveraging sqlite
 

Opened questions:
- We could rename the TUI "Conversation" column to "Name" or "Thread"
that would feel more accurate. Could be a fast-follow if we implement
auto-naming as it'll always be a name instead?

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

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+use std::collections::HashSet;
 use std::fs::File;
 use std::io::Read;
 use std::io::Seek;
@@ -8,6 +10,7 @@ use std::path::PathBuf;
 use codex_protocol::ThreadId;
 use serde::Deserialize;
 use serde::Serialize;
+use tokio::io::AsyncBufReadExt;
 use tokio::io::AsyncWriteExt;
 
 const SESSION_INDEX_FILE: &str = "session_index.jsonl";
@@ -76,6 +79,38 @@ pub async fn find_thread_name_by_id(
     Ok(entry.map(|entry| entry.thread_name))
 }
 
+/// Find the latest thread names for a batch of thread ids.
+pub async fn find_thread_names_by_ids(
+    codex_home: &Path,
+    thread_ids: &HashSet<ThreadId>,
+) -> std::io::Result<HashMap<ThreadId, String>> {
+    let path = session_index_path(codex_home);
+    if thread_ids.is_empty() || !path.exists() {
+        return Ok(HashMap::new());
+    }
+
+    let file = tokio::fs::File::open(&path).await?;
+    let reader = tokio::io::BufReader::new(file);
+    let mut lines = reader.lines();
+    let mut names = HashMap::with_capacity(thread_ids.len());
+
+    while let Some(line) = lines.next_line().await? {
+        let trimmed = line.trim();
+        if trimmed.is_empty() {
+            continue;
+        }
+        let Ok(entry) = serde_json::from_str::<SessionIndexEntry>(trimmed) else {
+            continue;
+        };
+        let name = entry.thread_name.trim();
+        if !name.is_empty() && thread_ids.contains(&entry.id) {
+            names.insert(entry.id, name.to_string());
+        }
+    }
+
+    Ok(names)
+}
+
 /// Find the most recently updated thread id for a thread name, if any.
 pub async fn find_thread_id_by_name(
     codex_home: &Path,
@@ -197,6 +232,8 @@ where
 mod tests {
     use super::*;
     use pretty_assertions::assert_eq;
+    use std::collections::HashMap;
+    use std::collections::HashSet;
     use tempfile::TempDir;
     fn write_index(path: &Path, lines: &[SessionIndexEntry]) -> std::io::Result<()> {
         let mut out = String::new();
@@ -279,6 +316,44 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn find_thread_names_by_ids_prefers_latest_entry() -> std::io::Result<()> {
+        let temp = TempDir::new()?;
+        let path = session_index_path(temp.path());
+        let id1 = ThreadId::new();
+        let id2 = ThreadId::new();
+        let lines = vec![
+            SessionIndexEntry {
+                id: id1,
+                thread_name: "first".to_string(),
+                updated_at: "2024-01-01T00:00:00Z".to_string(),
+            },
+            SessionIndexEntry {
+                id: id2,
+                thread_name: "other".to_string(),
+                updated_at: "2024-01-01T00:00:00Z".to_string(),
+            },
+            SessionIndexEntry {
+                id: id1,
+                thread_name: "latest".to_string(),
+                updated_at: "2024-01-02T00:00:00Z".to_string(),
+            },
+        ];
+        write_index(&path, &lines)?;
+
+        let mut ids = HashSet::new();
+        ids.insert(id1);
+        ids.insert(id2);
+
+        let mut expected = HashMap::new();
+        expected.insert(id1, "latest".to_string());
+        expected.insert(id2, "other".to_string());
+
+        let found = find_thread_names_by_ids(temp.path(), &ids).await?;
+        assert_eq!(found, expected);
+        Ok(())
+    }
+
     #[test]
     fn scan_index_finds_latest_match_among_mixed_entries() -> std::io::Result<()> {
         let temp = TempDir::new()?;