Add ThreadManager sample crate (#20141)

Summary:
- Add codex-thread-manager-sample, a one-shot binary that starts a
ThreadManager thread, submits a prompt, and prints the final assistant
output.
- Pass ThreadStore into ThreadManager::new and expose
thread_store_from_config for existing callsites.
- Build the sample Config directly with only --model and prompt inputs.

Verification:
- just fmt
- cargo check -p codex-thread-manager-sample -p codex-app-server -p
codex-mcp-server
- git diff --check

Tests: Not run per request.

codex-rs/app-server/src/codex_message_processor.rs

@@ -691,7 +691,7 @@ pub(crate) struct CodexMessageProcessorArgs {
     pub(crate) log_db: Option<LogDbLayer>,
 }
 
-fn configured_thread_store(config: &Config) -> Arc<dyn ThreadStore> {
+fn thread_store_from_config(config: &Config) -> Arc<dyn ThreadStore> {
     match &config.experimental_thread_store {
         ThreadStoreConfig::Local => Arc::new(configured_local_thread_store(config)),
         ThreadStoreConfig::Remote { endpoint } => Arc::new(RemoteThreadStore::new(endpoint)),
@@ -790,7 +790,7 @@ impl CodexMessageProcessor {
             outgoing: outgoing.clone(),
             analytics_events_client,
             arg0_paths,
-            thread_store: configured_thread_store(&config),
+            thread_store: thread_store_from_config(&config),
             config,
             config_manager,
             active_login: Arc::new(Mutex::new(None)),
@@ -2509,6 +2509,7 @@ impl CodexMessageProcessor {
         let imported_thread = self
             .thread_manager
             .start_thread_with_options(StartThreadOptions {
+                thread_store: thread_store_from_config(&config),
                 config,
                 initial_history: InitialHistory::Forked(rollout_items),
                 session_source: None,
@@ -2706,6 +2707,7 @@ impl CodexMessageProcessor {
             } = listener_task_context
                 .thread_manager
                 .start_thread_with_options(StartThreadOptions {
+                    thread_store: thread_store_from_config(&config),
                     config,
                     initial_history: match session_start_source
                         .unwrap_or(codex_app_server_protocol::ThreadStartSource::Startup)
@@ -4278,7 +4280,8 @@ impl CodexMessageProcessor {
         match self
             .thread_manager
             .resume_thread_with_history(
-                config,
+                config.clone(),
+                thread_store_from_config(&config),
                 thread_history,
                 self.auth_manager.clone(),
                 persist_extended_history,
@@ -4852,7 +4855,7 @@ impl CodexMessageProcessor {
 
             let fallback_model_provider = config.model_provider_id.clone();
             let instruction_sources = Self::instruction_sources_from_config(&config).await;
-            let fork_thread_store = configured_thread_store(&config);
+            let fork_thread_store = thread_store_from_config(&config);
 
             let NewThread {
                 thread_id,
@@ -4864,6 +4867,7 @@ impl CodexMessageProcessor {
                 .fork_thread_from_history(
                     ForkSnapshot::Interrupted,
                     config,
+                    fork_thread_store.clone(),
                     InitialHistory::Resumed(ResumedHistory {
                         conversation_id: source_thread_id,
                         history: history_items.clone(),
@@ -7084,7 +7088,8 @@ impl CodexMessageProcessor {
             .thread_manager
             .fork_thread(
                 ForkSnapshot::Interrupted,
-                config,
+                config.clone(),
+                thread_store_from_config(&config),
                 rollout_path,
                 /*persist_extended_history*/ false,
                 self.request_trace_context(request_id).await,