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-protocol/src/protocol/common.rs

@@ -203,6 +203,10 @@ client_request_definitions! {
         params: v2::ThreadArchiveParams,
         response: v2::ThreadArchiveResponse,
     },
+    ThreadUnsubscribe => "thread/unsubscribe" {
+        params: v2::ThreadUnsubscribeParams,
+        response: v2::ThreadUnsubscribeResponse,
+    },
     ThreadSetName => "thread/name/set" {
         params: v2::ThreadSetNameParams,
         response: v2::ThreadSetNameResponse,
@@ -822,6 +826,7 @@ server_notification_definitions! {
     ThreadStatusChanged => "thread/status/changed" (v2::ThreadStatusChangedNotification),
     ThreadArchived => "thread/archived" (v2::ThreadArchivedNotification),
     ThreadUnarchived => "thread/unarchived" (v2::ThreadUnarchivedNotification),
+    ThreadClosed => "thread/closed" (v2::ThreadClosedNotification),
     ThreadNameUpdated => "thread/name/updated" (v2::ThreadNameUpdatedNotification),
     ThreadTokenUsageUpdated => "thread/tokenUsage/updated" (v2::ThreadTokenUsageUpdatedNotification),
     TurnStarted => "turn/started" (v2::TurnStartedNotification),