Add an injectable attachment store to ThreadManager (#42634)

## What changed

- Add the `codex-attachment-store` crate with storage-neutral attachment metadata, references, errors, and an asynchronous persistence interface.
- Provide an inline implementation that preserves attachment bytes as media-typed base64 data URLs.
- Inject the store into `ThreadManager`, expose it to consumers, and retain inline storage as the default for existing entry points.

## Testing

- Verify inline storage round-trips binary, text, PNG, and JPEG data.
- Verify attachment debug output redacts URLs while retaining file IDs.

GitOrigin-RevId: 7688dcd3c89d7540ed2398f3e7c63881fbfcda97

codex-rs/thread-manager-sample/src/main.rs

@@ -66,6 +66,7 @@ use codex_core_api::init_state_db;
 use codex_core_api::install_image_generation_extension;
 use codex_core_api::item_event_to_server_notification;
 use codex_core_api::local_agent_graph_store_from_state_db;
+use codex_core_api::passthrough_image_store;
 use codex_core_api::resolve_installation_id;
 use codex_core_api::set_default_originator;
 use codex_core_api::thread_store_from_config;
@@ -149,6 +150,7 @@ async fn run_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
         Arc::new(extensions.build()),
         user_instructions_provider,
         /*analytics_events_client*/ None,
+        passthrough_image_store(),
         Arc::clone(&thread_store),
         local_agent_graph_store_from_state_db(state_db.as_ref()),
         installation_id,