Add a thread-store builder with host cleanup support (#58954)

## What changed

Replace `thread_store_from_config` with `ThreadStoreBuilder`, exported through `codex_core` and `codex_core_api`, and migrate callers to the builder.

Allow hosts to register `with_additional_cleanup` for permanent deletion in local stores. The callback runs after successful agent-board cleanup and before rollout deletion, under the existing deletion locks. Document that cleanup must be idempotent and must avoid thread-store operations that reacquire those locks. In-memory stores do not invoke the callback.

Preserve the existing backend selection, startup rollout migration, and compression behavior.

GitOrigin-RevId: f2853d7de06fe12ef29451b204cf9fcbb349c359

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

@@ -80,6 +80,7 @@ use codex_app_server_protocol::experimental_required_message;
 use codex_arg0::Arg0DispatchPaths;
 use codex_code_mode::CodeModeSessionProvider;
 use codex_core::ThreadManager;
+use codex_core::ThreadStoreBuilder;
 use codex_core::config::Config;
 use codex_core::config::ThreadStoreConfig;
 use codex_exec_server::EnvironmentManager;
@@ -317,7 +318,8 @@ impl MessageProcessor {
         // The thread store is intentionally process-scoped. Config reloads can
         // affect per-thread behavior, but they must not move newly started,
         // resumed, or forked threads to a different persistence backend/root.
-        let thread_store = codex_core::thread_store_from_config(config.as_ref(), state_db.clone());
+        let thread_store =
+            ThreadStoreBuilder::from_config(config.as_ref(), state_db.clone()).build();
         // Queue persistence requires SQLite, so in-memory thread stores and
         // app servers without a state database do not have a queue backend.
         let queue_store: Option<Arc<dyn QueueStore>> = match &config.experimental_thread_store {