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/core/src/lib.rs

@@ -125,6 +125,7 @@ pub use thread_manager::ThreadManager;
 pub use thread_manager::ThreadShutdownReport;
 pub use thread_manager::build_models_manager;
 pub use thread_manager::local_agent_graph_store_from_state_db;
+pub use thread_manager::passthrough_image_store;
 pub use thread_manager::thread_store_from_config;
 pub use tools::handlers::WaitForEnvironmentToolConfig;
 pub use web_search::web_search_action_detail;