## What changed
Add `StateRuntime::list_thread_attachment_threads` to find threads by exact `attachment_type` and `identity_key`, including threads without previews. Return thread IDs and archive state, with filters for all, archived, or non-archived threads.
Order results by thread ID and bind pagination cursors to the attachment identity and archive filter. Add a database index on `(attachment_type, identity_key, thread_id)` to support the lookup.
## Testing
Add coverage for exact identity matching, archive filters, pagination, rejection of cursors reused with different lookup parameters, and membership updates after attachment removal or thread deletion.
GitOrigin-RevId: 21f32eb0cb1845a82d740d67b374080b6e12eb03
