.NET: Add AgentWorkflowBuilder group chat (#861)

* Add AgentWorkflowBuilder group chat

And fix a variety of issues along the way:
- Use DateTime{Offset}.UtcNow rather than Now
- AIAgentHostExecutor shouldn't be publishing empty messages
- Sequential workflows should be flowing all history and not just the output from the previous agent as the input into the next agent
- Renamed some of the new agent workflow methods... still not super happy with the shape, though
- Simplified handoffs builder, e.g. using a hashset with a custom comparer instead of a dictionary
- Improved multi-service use by trying to change assistant->user role for messages created by other agents
- Changed MessageMerger to rely on M.E.AI's coalescing more and to avoid empty contents / text
- Ensured that messages from ChatClientAgent include MessageId and CreatedAt timestamps
- Avoided including instructions for agents in a handoff workflow that don't have any handoffs
- Removed the unnecessary end function in handoffs
- Improved naming of executors to include agent name for debuggability
- Use "N" formatting with Guid.ToString everywhere, to avoid the unnecessary extra dash character which is also not valid in various places (like function tool names)
- Replace `params T[]` with `params IEnumerable<T>` to make public APIs more flexible in what they consume

* Address feedback

- Fix unintentional provider change in sample

dotnet/tests/Microsoft.Extensions.AI.Agents.Abstractions.UnitTests/ChatMessageStoreTests.cs

@@ -78,7 +78,7 @@ public class ChatMessageStoreTests
     private sealed class TestChatMessageStore : ChatMessageStore
     {
         public override Task<IEnumerable<ChatMessage>> GetMessagesAsync(CancellationToken cancellationToken = default)
-            => Task.FromResult<IEnumerable<ChatMessage>>(Array.Empty<ChatMessage>());
+            => Task.FromResult<IEnumerable<ChatMessage>>([]);
 
         public override Task AddMessagesAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken = default)
             => Task.CompletedTask;