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/app-server/src/message_processor_tracing_tests.rs

@@ -32,7 +32,6 @@ use codex_config::CloudRequirementsLoader;
 use codex_config::LoaderOverrides;
 use codex_core::config::Config;
 use codex_core::config::ConfigBuilder;
-use codex_core::init_state_db_from_config;
 use codex_exec_server::EnvironmentManager;
 use codex_feedback::CodexFeedback;
 use codex_login::AuthManager;
@@ -282,9 +281,6 @@ async fn build_test_processor(
         outgoing_tx,
         analytics_events_client.clone(),
     ));
-    let state_db = init_state_db_from_config(config.as_ref())
-        .await
-        .expect("tracing test processor requires state db");
     let processor = Arc::new(MessageProcessor::new(MessageProcessorArgs {
         outgoing,
         analytics_events_client,
@@ -294,7 +290,7 @@ async fn build_test_processor(
         environment_manager: Arc::new(EnvironmentManager::default_for_tests()),
         feedback: CodexFeedback::new(),
         log_db: None,
-        state_db,
+        state_db: None,
         config_warnings: Vec::new(),
         session_source: SessionSource::VSCode,
         auth_manager,