fix: Duplicate CallIds cause Handoff Message Filtering to fail

Some providers, e.g. Gemini, do not use the CallId mechanism to disambiguate simultaneous function calls. This can result in message lists containing multiple turn to fail to filter properly.

The fix is to take advantage of the expectation that Handoff Orchestration is a "single-speaker" flow, which only has a single active AIAgent per "turn" and an agent's turn is not finished until all outstanding function calls are finished.

This allows us to expect that any ambiguous-CallId FunctionCallContent are either in separate turns or will have had a response before the next issued call with the same Id.

dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffAgentExecutor.cs

@@ -426,7 +426,7 @@ internal sealed class HandoffAgentExecutor :
                     {
                         AgentId = this._agent.Id,
                         AuthorName = this._agent.Name ?? this._agent.Id,
-                        Contents = [new FunctionResultContent(handoffRequest.CallId, "Transferred.")],
+                        Contents = [CreateHandoffResult(handoffRequest.CallId)],
                         CreatedAt = DateTimeOffset.UtcNow,
                         MessageId = Guid.NewGuid().ToString("N"),
                         Role = ChatRole.Tool,
@@ -459,4 +459,6 @@ internal sealed class HandoffAgentExecutor :
              ? this._handoffFunctionToAgentId.TryGetValue(requestedHandoff, out string? targetId) ? targetId : null
              : null;
     }
+
+    internal static FunctionResultContent CreateHandoffResult(string requestCallId) => new(requestCallId, "Transferred.");
 }