feat(harness): add chat application use case

src/main/java/com/superbiz/agent/harness/application/SystemChatContent.java

@@ -0,0 +1,22 @@
+package com.superbiz.agent.harness.application;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+public record SystemChatContent(@JsonProperty("answer") String answer)
+        implements ChatApplicationContent {
+
+    public SystemChatContent {
+        requireText(answer, "answer");
+    }
+
+    @Override
+    public ChatContentType contentType() {
+        return ChatContentType.SYSTEM_CHAT;
+    }
+
+    private static void requireText(String value, String name) {
+        if (value == null || value.isBlank()) {
+            throw new IllegalArgumentException(name + " must not be blank");
+        }
+    }
+}