feat(app-server): thread/unsubscribe API (#10954)

Adds a new v2 app-server API for a client to be able to unsubscribe to a
thread:
- New RPC method: `thread/unsubscribe`
- New server notification: `thread/closed`

Today clients can start/resume/archive threads, but there wasn’t a way
to explicitly unload a live thread from memory without archiving it.
With `thread/unsubscribe`, a client can indicate it is no longer
actively working with a live Thread. If this is the only client
subscribed to that given thread, the thread will be automatically closed
by app-server, at which point the server will send `thread/closed` and
`thread/status/changed` with `status: notLoaded` notifications.

This gives clients a way to prevent long-running app-server processes
from accumulating too many thread (and related) objects in memory.

Closed threads will also be removed from `thread/loaded/list`.

codex-rs/app-server/src/message_processor.rs

@@ -87,7 +87,7 @@ impl ExternalAuthRefresher for ExternalAuthRefreshBridge {
 
         let (request_id, rx) = self
             .outgoing
-            .send_request_with_id(ServerRequestPayload::ChatgptAuthTokensRefresh(params))
+            .send_request(ServerRequestPayload::ChatgptAuthTokensRefresh(params))
             .await;
 
         let result = match timeout(EXTERNAL_AUTH_REFRESH_TIMEOUT, rx).await {