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/Cargo.toml

@@ -25,6 +25,7 @@ codex-api = { workspace = true }
 codex-app-server-protocol = { workspace = true }
 codex-apply-patch = { workspace = true }
 codex-async-utils = { workspace = true }
+codex-attachment-store = { workspace = true }
 codex-client = { workspace = true }
 codex-code-mode = { workspace = true }
 codex-connectors = { workspace = true }