core: add extra metadata field to Thread struct (#29675)

# Summary

Adds a field Thread.extras that can be used to hold arbitrary metadata
specific to a given thread.

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -4156,6 +4156,7 @@ pub(crate) fn thread_from_stored_thread(
     let thread_id = thread.thread_id.to_string();
     let thread = Thread {
         id: thread_id.clone(),
+        extra: None,
         session_id: thread_id,
         forked_from_id: thread.forked_from_id.map(|id| id.to_string()),
         parent_thread_id: thread.parent_thread_id.map(|id| id.to_string()),
@@ -4366,6 +4367,7 @@ fn build_thread_from_snapshot(
     let now = time::OffsetDateTime::now_utc().unix_timestamp();
     Thread {
         id: thread_id.to_string(),
+        extra: None,
         session_id,
         forked_from_id: None,
         parent_thread_id: config_snapshot.parent_thread_id.map(|id| id.to_string()),

codex-rs/app-server/src/request_processors/thread_resume_redaction.rs

@@ -183,6 +183,7 @@ mod tests {
     fn test_thread(items: Vec<ThreadItem>) -> Thread {
         Thread {
             id: "thread-1".to_string(),
+            extra: None,
             session_id: "session-1".to_string(),
             forked_from_id: None,
             parent_thread_id: None,

codex-rs/app-server/src/request_processors/thread_summary.rs

@@ -314,6 +314,7 @@ pub(crate) fn summary_to_thread(
     let thread_id = conversation_id.to_string();
     Thread {
         id: thread_id.clone(),
+        extra: None,
         session_id: thread_id,
         forked_from_id: None,
         parent_thread_id: None,

codex-rs/app-server/src/thread_status.rs

@@ -889,6 +889,7 @@ mod tests {
     fn test_thread(thread_id: &str, source: codex_app_server_protocol::SessionSource) -> Thread {
         Thread {
             id: thread_id.to_string(),
+            extra: None,
             session_id: thread_id.to_string(),
             forked_from_id: None,
             parent_thread_id: None,