.NET: How can I provide a CustomMessageStore to a WorkflowAgent?

@peibekwe is already working on this.

Since Jun 26, 2026.

I have a workflow that I want to use as an agent:

```csharp
builder.AddAIAgent("group-chat", (sp, key) =>
{
var documentAgent = sp.GetRequiredKeyedService("document-management-agent");
var financialAgent = sp.GetRequiredKeyedService("financial-analysis-agent");

Workflow workflow =
AgentWorkflowBuilder
.CreateGroupChatBuilderWith(agents =>
new RoundRobinGroupChatManager(agents)
{
MaximumIterationCount = 2
})
.AddParticipants(documentAgent, financialAgent)
.Build();

return workflow.AsAgentAsync(name: key).GetAwaiter().GetResult();
});
```

Since this agent is hosted in an API, I want to handle its conversation history in a CustomMessageStore (e.g. CosmosDB). I didn't find a way to provide the CustomMessageStore as I do with a ChatClientAgent:

```csharp
ChatClientAgentOptions options = new ()
{
Id = Guid.NewGuid().ToString(),
Name = "myagent",
Instructions = @"A friendly AI assistant"
Description = "A friendly AI assistant",
ChatOptions = new ChatOptions
{
Tools = [.. documentTools],
},

ChatMessageStoreFactory = ctx =>
{
// Create a new chat message store for this agent that stores the messages in a cosmos store.
return new CustomMessageStore(
sp.GetRequiredService(),
ctx.SerializedState,
ctx.JsonSerializerOptions
);
}
};

var agent = new ChatClientAgent(instrumentedChatClient, options);
```