fix(ai): remove the expanded Surprise Me divider

Hide the close button's left border when the Surprise Me card is expanded.
Keep the original divider in the first-time card and preserve button
dimensions and behavior.

Verified both layouts in the connected Linux app and compiled the LESS
stylesheet successfully. No test suites were added or run for this
CSS-only change.

Author: abose

src/styles/Extn-AIChatPanel.less

@@ -1293,6 +1293,10 @@
     }
 
     .ai-start-surprise-open {
+        .ai-start-surprise-dismiss {
+            border-left: 0;
+        }
+
         .ai-start-surprise-choice {
             display: block;
             animation: ai-start-choice-open 0.22s ease-out;