Revert state DB injection and agent graph store (#21481)

## Why

Reverts #20689 to restore the previous optional state DB plumbing. The
conflict resolution keeps the newer installation ID and session/thread
identity changes that landed after #20689, while removing the mandatory
state DB and agent graph store dependency from ThreadManager
construction.

## What changed

- Restored `Option<StateDbHandle>` through app-server, MCP server,
prompt debug, and test entry points.
- Removed the `codex-core` dependency on `codex-agent-graph-store` and
reverted descendant lookup back to the existing state DB path when
available.
- Kept newer `installation_id` forwarding by passing it beside the
optional DB handle.
- Kept local thread-name updates working when the optional state DB
handle is absent.

## Validation

- `git diff --check`
- `cargo test -p codex-thread-store`
- `cargo test -p codex-state -p codex-rollout -p
codex-app-server-protocol`
- Attempted `env CARGO_INCREMENTAL=0 cargo test -p codex-core -p
codex-app-server -p codex-app-server-client -p codex-mcp-server -p
codex-thread-manager-sample -p codex-tui`; blocked locally by a rustc
ICE while compiling `v8 v146.4.0` with `rustc 1.93.0 (254b59607
2026-01-19)` on `aarch64-apple-darwin`.

codex-rs/mcp-server/src/message_processor.rs

@@ -2,10 +2,9 @@ use std::collections::HashMap;
 use std::sync::Arc;
 
 use codex_arg0::Arg0DispatchPaths;
+use codex_core::StateDbHandle;
 use codex_core::ThreadManager;
-use codex_core::agent_graph_store_from_state_db;
 use codex_core::config::Config;
-use codex_core::init_state_db_from_config;
 use codex_core::thread_store_from_config;
 use codex_exec_server::EnvironmentManager;
 use codex_login::AuthManager;
@@ -55,35 +54,32 @@ impl MessageProcessor {
         arg0_paths: Arg0DispatchPaths,
         config: Arc<Config>,
         environment_manager: Arc<EnvironmentManager>,
+        state_db: Option<StateDbHandle>,
         installation_id: String,
-    ) -> Option<Self> {
+    ) -> Self {
         let outgoing = Arc::new(outgoing);
         let auth_manager = AuthManager::shared_from_config(
             config.as_ref(),
             /*enable_codex_api_key_env*/ false,
         )
         .await;
-        let state_db = init_state_db_from_config(config.as_ref()).await?;
-        let thread_store = thread_store_from_config(config.as_ref(), state_db.clone());
-        let agent_graph_store = agent_graph_store_from_state_db(state_db.clone());
         let thread_manager = Arc::new(ThreadManager::new(
             config.as_ref(),
             auth_manager,
             SessionSource::Mcp,
             environment_manager,
             /*analytics_events_client*/ None,
-            state_db,
-            thread_store,
-            agent_graph_store,
+            thread_store_from_config(config.as_ref(), state_db.clone()),
+            state_db.clone(),
             installation_id,
         ));
-        Some(Self {
+        Self {
             outgoing,
             initialized: false,
             arg0_paths,
             thread_manager,
             running_requests_id_to_codex_uuid: Arc::new(Mutex::new(HashMap::new())),
-        })
+        }
     }
 
     pub(crate) async fn process_request(&mut self, request: JsonRpcRequest<ClientRequest>) {