Add appearance metadata to thread sections (#37898)

## What changed

- Add optional `icon` and `color` appearance fields to custom thread sections and expose them through the app-server protocol.
- Persist appearance metadata in SQLite and include it when listing sections or returning threads.
- Let `threadSection/update` preserve an omitted appearance, clear it with `null`, or replace it with a new value.
- Reject appearance fields larger than 64 bytes.

## Testing

- Cover protocol compatibility, persistence across restart, update and clear behavior, and field-length validation.

GitOrigin-RevId: 8f55ce4f42f8bd17aa5416848a85845f681953df

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

@@ -4,6 +4,18 @@
     "ThreadSection": {
       "description": "An independently persisted, user-visible thread section.",
       "properties": {
+        "appearance": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ThreadSectionAppearance"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "description": "Optional appearance synchronized across clients."
+        },
         "id": {
           "description": "Opaque UUIDv7 identity that remains stable when the section is renamed.",
           "type": "string"
@@ -18,6 +30,24 @@
         "name"
       ],
       "type": "object"
+    },
+    "ThreadSectionAppearance": {
+      "description": "Extensible visual presentation for a custom thread section.",
+      "properties": {
+        "color": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "icon": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "type": "object"
     }
   },
   "description": "The independently persisted section created by the server.",