Add thread/unarchive to restore archived rollouts (#9843)

## Summary
- Adds a new `thread/unarchive` RPC to move archived thread rollouts
back into the active `sessions/` tree.

## What changed
- **Protocol**
  - Adds `thread/unarchive` request/response types and wiring.
- **Server**
  - Implements `thread_unarchive` in the app server.
  - Validates the archived rollout path and thread ID.
- Restores the rollout to `sessions/YYYY/MM/DD/...` based on the rollout
filename timestamp.
- **Core**
- Adds `find_archived_thread_path_by_id_str` helper for archived
rollouts.
- **Docs**
  - Documents the new RPC and usage example.
- **Tests**
  - Adds an end-to-end server test that:
    1) starts a thread,
    2) archives it,
    3) unarchives it,
    4) asserts the file is restored to `sessions/`.

## How to use
```json
{ "method": "thread/unarchive", "id": 24, "params": { "threadId": "<thread-id>" } }
```

## Author Codex Session

`codex resume 019bf158-54b6-7960-a696-9d85df7e1bc1` (soon I'll make this
kind of session UUID forkable by anyone with the right
`session_object_storage_url` line in their config, but for now just
pasting it here for my reference)

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -18,5 +18,6 @@ mod thread_read;
 mod thread_resume;
 mod thread_rollback;
 mod thread_start;
+mod thread_unarchive;
 mod turn_interrupt;
 mod turn_start;