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/codex_app_server_protocol.schemas.json

@@ -20922,6 +20922,18 @@
       "ThreadSection": {
         "description": "An independently persisted, user-visible thread section.",
         "properties": {
+          "appearance": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/ThreadSectionAppearance"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "default": null,
+            "description": "Optional appearance synchronized across clients."
+          },
           "id": {
             "description": "Opaque UUIDv7 identity that remains stable when the section is renamed.",
             "type": "string"
@@ -20937,10 +20949,39 @@
         ],
         "type": "object"
       },
+      "ThreadSectionAppearance": {
+        "description": "Extensible visual presentation for a custom thread section.",
+        "properties": {
+          "color": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "icon": {
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "type": "object"
+      },
       "ThreadSectionCreateParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "description": "Parameters for creating an independently persisted thread section.",
         "properties": {
+          "appearance": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/ThreadSectionAppearance"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "default": null
+          },
           "name": {
             "description": "The user-visible name of the section.",
             "type": "string"
@@ -21074,6 +21115,17 @@
         "$schema": "http://json-schema.org/draft-07/schema#",
         "description": "Parameters for updating an independently persisted thread section.",
         "properties": {
+          "appearance": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/ThreadSectionAppearance"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "description": "Omit to preserve appearance, use `null` to clear it, or provide a replacement."
+          },
           "name": {
             "description": "The updated user-visible name of the section.",
             "type": "string"