[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/thread-manager-sample/src/main.rs

@@ -59,6 +59,7 @@ use codex_core_api::empty_extension_registry;
 use codex_core_api::find_codex_home;
 use codex_core_api::init_state_db;
 use codex_core_api::item_event_to_server_notification;
+use codex_core_api::local_agent_graph_store_from_state_db;
 use codex_core_api::resolve_installation_id;
 use codex_core_api::set_default_originator;
 use codex_core_api::thread_store_from_config;
@@ -133,7 +134,7 @@ async fn run_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
         user_instructions_provider,
         /*analytics_events_client*/ None,
         Arc::clone(&thread_store),
-        state_db,
+        local_agent_graph_store_from_state_db(state_db.as_ref()),
         installation_id,
         /*attestation_provider*/ None,
         /*external_time_provider*/ None,