.NET: Fix Error 404 Agent Hosted MCP (#3678)

* Initial plan

* Fix issue #3195: Handle empty Version and ID in Azure AI agent responses

This fix addresses the issue where hosted MCP agents (like AgentWithHostedMCP)
fail with "ID cannot be null or empty (Parameter 'id')" error when deployed
to Azure AI Foundry.

Changes:
- Add CreateAgentReference helper method in AzureAIProjectChatClient that defaults
  empty version to "latest"
- Update CreateChatClientAgentOptions to generate a fallback ID from name and version
  when AgentVersion.Id is null or empty
- Add GetAgentVersionResponseJsonWithEmptyVersion and GetAgentResponseJsonWithEmptyVersion
  test data methods
- Add unit tests for empty version handling scenarios

Co-authored-by: rogerbarreto <19890735+rogerbarreto@users.noreply.github.com>

* Address code review feedback: improve documentation and test comments

Co-authored-by: rogerbarreto <19890735+rogerbarreto@users.noreply.github.com>

* Address PR review: Use IsNullOrWhiteSpace and add whitespace unit tests

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

dotnet/src/Microsoft.Agents.AI.AzureAI/AzureAIProjectChatClient.cs

@@ -64,13 +64,27 @@ internal sealed class AzureAIProjectChatClient : DelegatingChatClient
     internal AzureAIProjectChatClient(AIProjectClient aiProjectClient, AgentVersion agentVersion, ChatOptions? chatOptions)
         : this(
               aiProjectClient,
-              new AgentReference(Throw.IfNull(agentVersion).Name, agentVersion.Version),
+              CreateAgentReference(Throw.IfNull(agentVersion)),
               (agentVersion.Definition as PromptAgentDefinition)?.Model,
               chatOptions)
     {
         this._agentVersion = agentVersion;
     }
 
+    /// <summary>
+    /// Creates an <see cref="AgentReference"/> from an <see cref="AgentVersion"/>.
+    /// Uses the agent version's version if available, otherwise defaults to "latest".
+    /// </summary>
+    /// <param name="agentVersion">The agent version to create a reference from.</param>
+    /// <returns>An <see cref="AgentReference"/> for the specified agent version.</returns>
+    private static AgentReference CreateAgentReference(AgentVersion agentVersion)
+    {
+        // If the version is null, empty, or whitespace, use "latest" as the default.
+        // This handles cases where hosted agents (like MCP agents) may not have a version assigned.
+        var version = string.IsNullOrWhiteSpace(agentVersion.Version) ? "latest" : agentVersion.Version;
+        return new AgentReference(agentVersion.Name, version);
+    }
+
     /// <inheritdoc/>
     public override object? GetService(Type serviceType, object? serviceKey = null)
     {

dotnet/src/Microsoft.Agents.AI.AzureAI/AzureAIProjectChatClientExtensions.cs

@@ -543,9 +543,16 @@ public static partial class AzureAIProjectChatClientExtensions
             }
         }
 
+        // Use the agent version's ID if available, otherwise generate one from name and version.
+        // This handles cases where hosted agents (like MCP agents) may not have an ID assigned.
+        var version = string.IsNullOrWhiteSpace(agentVersion.Version) ? "latest" : agentVersion.Version;
+        var agentId = string.IsNullOrWhiteSpace(agentVersion.Id)
+            ? $"{agentVersion.Name}:{version}"
+            : agentVersion.Id;
+
         var agentOptions = new ChatClientAgentOptions()
         {
-            Id = agentVersion.Id,
+            Id = agentId,
             Name = agentVersion.Name,
             Description = agentVersion.Description,
         };