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/ThreadAttachmentRemoveParams.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Parameters for deleting an attachment by its stable thread-local identity.",
+  "properties": {
+    "attachmentType": {
+      "type": "string"
+    },
+    "identityKey": {
+      "type": "string"
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "attachmentType",
+    "identityKey",
+    "threadId"
+  ],
+  "title": "ThreadAttachmentRemoveParams",
+  "type": "object"
+}