.NET: Add TTLs to durable agent sessions (#2679)

* .NET: Add TTLs to durable agent sessions

* Remove unnecessary async

* PR feedback: clarify UTC

* PR feedback: limit minimum signal delay to <= 5 minutes

* PR feedback: Fix TTL disablement

* Linter: use auto-property

* Fix build break from OpenAI SDK change

* Updated CHANGELOG.md

* PR feedback

* Reduce default TTL to 14 days to work around DTS bug

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

@@ -17,6 +17,13 @@ internal sealed class DurableAgentStateData
     [JsonPropertyName("conversationHistory")]
     public IList<DurableAgentStateEntry> ConversationHistory { get; init; } = [];
 
+    /// <summary>
+    /// Gets or sets the expiration time (UTC) for this agent entity.
+    /// If the entity is idle beyond this time, it will be automatically deleted.
+    /// </summary>
+    [JsonPropertyName("expirationTimeUtc")]
+    public DateTime? ExpirationTimeUtc { get; set; }
+
     /// <summary>
     /// Gets any additional data found during deserialization that does not map to known properties.
     /// </summary>