[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/core/src/tools/handlers/multi_agents_tests.rs

@@ -4,6 +4,7 @@ use crate::config::AgentRoleConfig;
 use crate::config::DEFAULT_AGENT_MAX_DEPTH;
 use crate::function_tool::FunctionCallError;
 use crate::init_state_db;
+use crate::local_agent_graph_store_from_state_db;
 use crate::session::step_context::StepContext;
 use crate::session::tests::make_session_and_context;
 use crate::session_prefix::format_inter_agent_completion_message;
@@ -4267,7 +4268,7 @@ async fn tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtr
         Arc::new(crate::test_support::EmptyUserInstructionsProvider),
         /*analytics_events_client*/ None,
         thread_store_from_config(&config, state_db.clone()),
-        state_db.clone(),
+        local_agent_graph_store_from_state_db(state_db.as_ref()),
         "11111111-1111-4111-8111-111111111111".to_string(),
         /*attestation_provider*/ None,
         /*external_time_provider*/ None,