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/schema/json/codex_app_server_protocol.schemas.json

@@ -475,6 +475,30 @@
           "title": "Thread/archiveRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/RequestId"
+            },
+            "method": {
+              "enum": [
+                "thread/unsubscribe"
+              ],
+              "title": "Thread/unsubscribeRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/ThreadUnsubscribeParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Thread/unsubscribeRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -5903,6 +5927,26 @@
           "title": "Thread/unarchivedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "thread/closed"
+              ],
+              "title": "Thread/closedNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/ThreadClosedNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "Thread/closedNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -12543,6 +12587,19 @@
         "title": "ThreadArchivedNotification",
         "type": "object"
       },
+      "ThreadClosedNotification": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "threadId": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "threadId"
+        ],
+        "title": "ThreadClosedNotification",
+        "type": "object"
+      },
       "ThreadCompactStartParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "properties": {
@@ -14057,6 +14114,40 @@
         "title": "ThreadUnarchivedNotification",
         "type": "object"
       },
+      "ThreadUnsubscribeParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "threadId": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "threadId"
+        ],
+        "title": "ThreadUnsubscribeParams",
+        "type": "object"
+      },
+      "ThreadUnsubscribeResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "status": {
+            "$ref": "#/definitions/v2/ThreadUnsubscribeStatus"
+          }
+        },
+        "required": [
+          "status"
+        ],
+        "title": "ThreadUnsubscribeResponse",
+        "type": "object"
+      },
+      "ThreadUnsubscribeStatus": {
+        "enum": [
+          "notLoaded",
+          "notSubscribed",
+          "unsubscribed"
+        ],
+        "type": "string"
+      },
       "TokenUsageBreakdown": {
         "properties": {
           "cachedInputTokens": {