fix: only project handoff tool calls as FunctionCallContent

AryxCopilotAgent was converting ALL Copilot tool requests (ask_user,
MCP tools, etc.) into FunctionCallContent. The Agent Framework's
AIAgentHostExecutor tracks every FunctionCallContent as an outstanding
request.  Since SDK-handled tools like ask_user never produce a
matching FunctionResultContent, HasOutstandingRequests stayed true and
TurnToken was never emitted — stalling group-chat advancement after
an ask_user interaction.

Now only handoff_to_* tool requests are projected as FunctionCallContent.
All other tool calls remain invisible to the Agent Framework executor,
matching the upstream GitHubCopilotAgent behaviour.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/tests/Aryx.AgentHost.Tests/CopilotAgentBundleTests.cs

@@ -125,6 +125,24 @@ public sealed class CopilotAgentBundleTests
         Assert.Equal("frontend specialist", functionCall.Arguments["reasonForHandoff"]?.ToString());
     }
 
+    [Fact]
+    public void ConvertToolRequestsToFunctionCalls_SkipsNonHandoffToolCalls()
+    {
+        AssistantMessageDataToolRequestsItem[] toolRequests =
+        {
+            new() { ToolCallId = "call-001", Name = "ask_user" },
+            new() { ToolCallId = "call-002", Name = "web_fetch" },
+            new() { ToolCallId = "call-003", Name = "handoff_to_reviewer" },
+            new() { ToolCallId = "call-004", Name = "grep" },
+        };
+
+        IReadOnlyList<FunctionCallContent> result = AryxCopilotAgent.ConvertToolRequestsToFunctionCalls(toolRequests);
+
+        FunctionCallContent single = Assert.Single(result);
+        Assert.Equal("call-003", single.CallId);
+        Assert.Equal("handoff_to_reviewer", single.Name);
+    }
+
     private static AIFunction CreateTool()
     {
         ToolTarget target = new();