[codex] Inject agent graph store into ThreadManager (#29736)

Pick up the AgentGraphStore migration.

- Inject an explicit optional agent graph store into `ThreadManager` 
- Move all calls to spawn, close, recursive resume, and
subtree/archive/delete/feedback traversal through it
- Keep using  `LocalAgentGraphStore` when SQLite is available

This required some changes to the interface to deal with futures:

- The interface now matches `ThreadStore`'s object-safe pattern by
returning a boxed `AgentGraphStoreFuture` directly, allowing
`ThreadManager` to hold `Arc<dyn AgentGraphStore>`

*Slight behavior change!* Unfiltered subtree enumeration now performs a
single all-status breadth-first traversal, so a closed grandchild
beneath an open edge is included; the previous Open-then-Closed
traversals could not cross mixed-status paths and silently omitted it.

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

@@ -102,27 +102,7 @@ impl FeedbackRequestProcessor {
                         warn!(
                             "failed to list feedback subtree for thread_id={conversation_id}: {err}"
                         );
-                        let mut thread_ids = vec![conversation_id];
-                        if let Some(state_db_ctx) = state_db_ctx.as_ref() {
-                            for status in [
-                                codex_state::DirectionalThreadSpawnEdgeStatus::Open,
-                                codex_state::DirectionalThreadSpawnEdgeStatus::Closed,
-                            ] {
-                                match state_db_ctx
-                                    .list_thread_spawn_descendants_with_status(
-                                        conversation_id,
-                                        status,
-                                    )
-                                    .await
-                                {
-                                    Ok(descendant_ids) => thread_ids.extend(descendant_ids),
-                                    Err(err) => warn!(
-                                        "failed to list persisted feedback subtree for thread_id={conversation_id}: {err}"
-                                    ),
-                                }
-                            }
-                        }
-                        thread_ids
+                        vec![conversation_id]
                     }
                 },
                 None => Vec::new(),