Expose last message timestamp on sessions (#9961)

Co-authored-by: Douwe M Osinga <douwe@sidewalklabs.com>

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

@@ -1414,6 +1414,7 @@ export type Session = {
     extension_data: ExtensionData;
     goose_mode?: GooseMode;
     id: string;
+    last_message_at?: string | null;
     last_message_snippet?: string | null;
     message_count: number;
     model_config?: ModelConfig | null;