app-server: expose loaded thread status via read/list and notifications (#11786)

Motivation
- Today, a newly connected client has no direct way to determine the
current runtime status of threads from read/list responses alone.
- This forces clients to infer state from transient events, which can
lead to stale or inconsistent UI when reconnecting or attaching late.

Changes
- Add `status` to `thread/read` responses.
- Add `statuses` to `thread/list` responses.
- Emit `thread/status/changed` notifications with `threadId` and the new
status.
- Track runtime status for all loaded threads and default unknown
threads to `idle`.
- Update protocol/docs/tests/schema fixtures for the revised API.

Testing
- Validated protocol API changes with automated protocol tests and
regenerated schema/type fixtures.
- Validated app-server behavior with unit and integration test suites,
including status transitions and notifications.

codex-rs/app-server-protocol/schema/json/v2/ThreadStatusChangedNotification.json

@@ -0,0 +1,101 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ThreadActiveFlag": {
+      "enum": [
+        "waitingOnApproval",
+        "waitingOnUserInput"
+      ],
+      "type": "string"
+    },
+    "ThreadStatus": {
+      "oneOf": [
+        {
+          "properties": {
+            "type": {
+              "enum": [
+                "notLoaded"
+              ],
+              "title": "NotLoadedThreadStatusType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "type"
+          ],
+          "title": "NotLoadedThreadStatus",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "type": {
+              "enum": [
+                "idle"
+              ],
+              "title": "IdleThreadStatusType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "type"
+          ],
+          "title": "IdleThreadStatus",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "type": {
+              "enum": [
+                "systemError"
+              ],
+              "title": "SystemErrorThreadStatusType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "type"
+          ],
+          "title": "SystemErrorThreadStatus",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "activeFlags": {
+              "items": {
+                "$ref": "#/definitions/ThreadActiveFlag"
+              },
+              "type": "array"
+            },
+            "type": {
+              "enum": [
+                "active"
+              ],
+              "title": "ActiveThreadStatusType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "activeFlags",
+            "type"
+          ],
+          "title": "ActiveThreadStatus",
+          "type": "object"
+        }
+      ]
+    }
+  },
+  "properties": {
+    "status": {
+      "$ref": "#/definitions/ThreadStatus"
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "status",
+    "threadId"
+  ],
+  "title": "ThreadStatusChangedNotification",
+  "type": "object"
+}