.NET: [Durable Agents] Filter empty AIContent from durable agent state responses (#4670)

* Filter empty AIContent from durable agent state responses

Prevent opaque AIContent objects (e.g., with only RawRepresentation set)
from being stored in durable entity state, where they serialize to empty
JSON payloads. Base AIContent instances are kept only if they have
Annotations or AdditionalProperties.

Fixes https://github.com/microsoft/agent-framework/issues/4481

* Update CHANGELOG.md and fix linter violation

dotnet/src/Microsoft.Agents.AI.DurableTask/State/DurableAgentStateResponse.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Text.Json.Serialization;
+using Microsoft.Extensions.AI;
 
 namespace Microsoft.Agents.AI.DurableTask.State;
 
@@ -28,7 +29,10 @@ internal sealed class DurableAgentStateResponse : DurableAgentStateEntry
         {
             CorrelationId = correlationId,
             CreatedAt = response.CreatedAt ?? response.Messages.Max(m => m.CreatedAt) ?? DateTimeOffset.UtcNow,
-            Messages = response.Messages.Select(DurableAgentStateMessage.FromChatMessage).ToList(),
+            Messages = response.Messages
+                .Where(HasSerializableContent)
+                .Select(DurableAgentStateMessage.FromChatMessage)
+                .ToList(),
             Usage = DurableAgentStateUsage.FromUsage(response.Usage)
         };
     }
@@ -46,4 +50,18 @@ internal sealed class DurableAgentStateResponse : DurableAgentStateEntry
             Usage = this.Usage?.ToUsageDetails(),
         };
     }
+
+    // Checks whether a ChatMessage has any content that will produce meaningful serialized data.
+    // Known derived AIContent types (TextContent, FunctionCallContent, etc.) are always serializable.
+    // Base AIContent instances only carry RawRepresentation (which is [JsonIgnore]), Annotations, and
+    // AdditionalProperties. We keep the message if any base AIContent has annotations or additional
+    // properties set. NOTE: if AIContent gains new serializable properties in the future, this check
+    // should be updated accordingly.
+    private static bool HasSerializableContent(ChatMessage message)
+    {
+        return message.Contents.Any(c =>
+            c.GetType() != typeof(AIContent) ||
+            c.Annotations?.Count > 0 ||
+            c.AdditionalProperties?.Count > 0);
+    }
 }