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/common/mcp_process.rs

@@ -53,6 +53,7 @@ use codex_app_server_protocol::ThreadReadParams;
 use codex_app_server_protocol::ThreadResumeParams;
 use codex_app_server_protocol::ThreadRollbackParams;
 use codex_app_server_protocol::ThreadStartParams;
+use codex_app_server_protocol::ThreadUnarchiveParams;
 use codex_app_server_protocol::TurnInterruptParams;
 use codex_app_server_protocol::TurnStartParams;
 use codex_core::default_client::CODEX_INTERNAL_ORIGINATOR_OVERRIDE_ENV_VAR;
@@ -365,6 +366,15 @@ impl McpProcess {
         self.send_request("thread/archive", params).await
     }
 
+    /// Send a `thread/unarchive` JSON-RPC request.
+    pub async fn send_thread_unarchive_request(
+        &mut self,
+        params: ThreadUnarchiveParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("thread/unarchive", params).await
+    }
+
     /// Send a `thread/rollback` JSON-RPC request.
     pub async fn send_thread_rollback_request(
         &mut self,