fix: add required topic to conversationHistory user turn in NUT fixture

The AiTestingDefinition ReturnsCheckoutSuite test-case 4 had a
conversationHistory user turn (index 0) with no <topic>. Server-side
validation now requires topic on every conversationHistory entry, so the
before-all deploy failed with "Test case 4, input 1, conversation
history 1: topic is required and cannot be blank". Add the topic to
match the agent turn.

Author: nico-pappagianis

test/mock-projects/agent-generate-template/force-app/main/default/aiTestingDefinitions/ReturnsCheckoutSuite.aiTestingDefinition-meta.xml

@@ -67,6 +67,7 @@
             <conversationHistory>
                 <role>user</role>
                 <message>I need help with my order</message>
+                <topic>identity_verification</topic>
                 <index>0</index>
             </conversationHistory>
             <conversationHistory>