feat(session): add opt-in ACP last message snippets (#9798)

Signed-off-by: Matt Toohey <contact@matttoohey.com>

ui/desktop/openapi.json

@@ -7979,6 +7979,10 @@
             "format": "int32",
             "nullable": true
           },
+          "last_message_snippet": {
+            "type": "string",
+            "nullable": true
+          },
           "message_count": {
             "type": "integer",
             "minimum": 0

ui/desktop/src/api/types.gen.ts

@@ -1340,6 +1340,7 @@ export type Session = {
     goose_mode?: GooseMode;
     id: string;
     input_tokens?: number | null;
+    last_message_snippet?: string | null;
     message_count: number;
     model_config?: ModelConfig | null;
     name: string;