Add app-server APIs for stored thread attachments (#44564)

## What changed

- Add `thread/attachment/add`, `thread/attachment/list`, and `thread/attachment/remove` to manage durable resource references without loading the owning thread or changing conversation history.
- Identify attachments by thread, `attachmentType`, and `identityKey`. Repeated adds return the existing attachment; repeated removals succeed without emitting another update. Listing supports cursor pagination.
- Broadcast `thread/attachment/updated` after creation or deletion, after responding to the requester. Serialize attachment mutations with thread lifecycle operations and reject stores that do not support attachments.
- Update protocol schemas, TypeScript and Python bindings, documentation, and TUI notification handling so attachment updates do not add untracked threads to the agent picker.

## Testing

Add coverage for unloaded threads, pagination, idempotent addition and removal, reattachment, invalid inputs, unsupported stores, response-before-notification ordering across clients, and TUI notification routing.

GitOrigin-RevId: 0d29cf903afd4e931a70a9cf96300475213b34aa

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

@@ -0,0 +1,52 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ThreadAttachment": {
+      "description": "An independently persisted attachment associated with a thread.",
+      "properties": {
+        "attachmentType": {
+          "type": "string"
+        },
+        "createdAt": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "id": {
+          "type": "string"
+        },
+        "identityKey": {
+          "type": "string"
+        },
+        "payload": true
+      },
+      "required": [
+        "attachmentType",
+        "createdAt",
+        "id",
+        "identityKey",
+        "payload"
+      ],
+      "type": "object"
+    }
+  },
+  "description": "One page of attachments associated with the requested thread.",
+  "properties": {
+    "data": {
+      "items": {
+        "$ref": "#/definitions/ThreadAttachment"
+      },
+      "type": "array"
+    },
+    "nextCursor": {
+      "type": [
+        "string",
+        "null"
+      ]
+    }
+  },
+  "required": [
+    "data"
+  ],
+  "title": "ThreadAttachmentListResponse",
+  "type": "object"
+}