Improve Response.messages field documentation

Co-authored-by: nikhilNava <211831449+nikhilNava@users.noreply.github.com>

Author: Copilot

libraries/microsoft-agents-a365-observability-core/microsoft_agents_a365/observability/core/models/response.py

@@ -9,4 +9,9 @@ class Response:
     """Response details from agent execution."""
 
     messages: list[str]
-    """The list of response messages."""
+    """The list of response messages from the agent.
+
+    Each message represents a text response generated by the agent during execution.
+    Messages are serialized to JSON format for OpenTelemetry span attributes.
+    An empty list is valid and represents no response messages.
+    """