Add transactional thread attachment mutations to the state runtime (#43949)

## What changed

- Replace the `ThreadArtifact` model and related exports with attachment terminology.
- Add `StateRuntime::add_thread_attachment` and `remove_thread_attachment` using SQLite transactions. Repeated additions for the same thread, attachment type, and identity key return the existing record without changing its payload or creation time.
- Enforce limits of 100 attachments per thread, 64 KiB per serialized payload, and 256 bytes each for nonblank attachment types and identity keys. Removal returns the deleted record or `NotFound` and frees capacity immediately. Both mutations reject unknown threads.

## Testing

Add tests for idempotency, thread isolation, removal outcomes, capacity reuse, invalid inputs, unknown threads, and concurrent additions creating exactly one record.

GitOrigin-RevId: a68fe5076832f071524f1b4fc87d08b96ac270b7

codex-rs/state/src/runtime.rs

@@ -52,6 +52,7 @@ mod remote_control;
 mod rollout_migration;
 #[cfg(test)]
 pub(crate) mod test_support;
+mod thread_attachments;
 mod thread_section_order;
 mod thread_sections;
 mod threads;