app-server: document thread and turn IDs are UUID7 (#27714)

It's actually a very nice property that these are UUID7s, so documenting
them so we think twice before changing it away from UUID7s in the
future.

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

@@ -865,6 +865,7 @@
           "description": "Optional Git metadata captured when the thread was created."
         },
         "id": {
+          "description": "Identifier for this thread. Codex-generated thread IDs are UUIDv7.",
           "type": "string"
         },
         "modelProvider": {
@@ -1823,6 +1824,7 @@
           "description": "Only populated when the Turn's status is failed."
         },
         "id": {
+          "description": "Identifier for this turn. Codex-generated turn IDs are UUIDv7.",
           "type": "string"
         },
         "items": {