[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/tests/common/test_codex.rs

@@ -608,7 +608,7 @@ impl TestCodexBuilder {
             user_instructions_provider,
             /*analytics_events_client*/ None,
             thread_store,
-            state_db.clone(),
+            codex_core::local_agent_graph_store_from_state_db(state_db.as_ref()),
             installation_id,
             /*attestation_provider*/ None,
             /*external_time_provider*/ self.external_time_provider.clone(),

codex-rs/core/tests/suite/client.rs

@@ -1464,7 +1464,7 @@ async fn prefers_apikey_when_config_prefers_apikey_even_with_chatgpt_tokens() {
         Arc::new(codex_core::test_support::EmptyUserInstructionsProvider),
         /*analytics_events_client*/ None,
         thread_store_from_config(&config, /*state_db*/ None),
-        /*state_db*/ None,
+        /*agent_graph_store*/ None,
         installation_id,
         /*attestation_provider*/ None,
         /*external_time_provider*/ None,