app-server: Emit thread archive/unarchive notifications (#12030)

* Add v2 server notifications `thread/archived` and `thread/unarchived`
with a `threadId` payload.
* Wire new events into `thread/archive` and `thread/unarchive` success
paths.
* Update app-server protocol/schema/docs accordingly.

Testing:
- Updated archive/unarchive end-to-end tests to verify both
notifications are emitted with the expected thread id payload.

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -6539,6 +6539,17 @@
       ],
       "type": "object"
     },
+    "ThreadArchivedNotification": {
+      "properties": {
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "threadId"
+      ],
+      "type": "object"
+    },
     "ThreadId": {
       "type": "string"
     },
@@ -7083,6 +7094,17 @@
       ],
       "type": "object"
     },
+    "ThreadUnarchivedNotification": {
+      "properties": {
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "threadId"
+      ],
+      "type": "object"
+    },
     "TokenUsage": {
       "properties": {
         "cached_input_tokens": {
@@ -8076,6 +8098,46 @@
       "title": "Thread/startedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/archived"
+          ],
+          "title": "Thread/archivedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ThreadArchivedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/archivedNotification",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/unarchived"
+          ],
+          "title": "Thread/unarchivedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ThreadUnarchivedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/unarchivedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {