feat: enhance message broadcasting to exclude sender's connection and improve IRC compliance

src/EchoHub.Server/Hubs/ChatHub.cs

@@ -103,7 +103,7 @@ public class ChatHub : Hub<IEchoHubClient>
     {
         try
         {
-            var error = await _chatService.SendMessageAsync(CurrentUserId, CurrentUsername, channelName, content);
+            var error = await _chatService.SendMessageAsync(CurrentUserId, CurrentUsername, channelName, content, Context.ConnectionId);
             if (error is not null)
                 await Clients.Caller.Error(error);
         }

src/EchoHub.Server/Services/ChatService.cs

@@ -151,7 +151,7 @@ public class ChatService : IChatService
         _logger.LogInformation("{User} left channel '{Channel}'", username, channelName);
     }
 
-    public async Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content)
+    public async Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content, string? originConnectionId = null)
     {
         channelName = channelName.ToLowerInvariant().Trim();
 
@@ -240,7 +240,7 @@ public class ChatService : IChatService
             Embeds: embeds,
             SenderDisplayName: sender?.DisplayName);
 
-        await BroadcastToAllAsync(b => b.SendMessageToChannelAsync(channelName, messageDto));
+        await BroadcastToAllAsync(b => b.SendMessageToChannelAsync(channelName, messageDto, originConnectionId));
 
         _logger.LogDebug("{User} sent message in '{Channel}'", username, channelName);
         return null;

src/EchoHub.Server/Services/SignalRBroadcaster.cs

@@ -20,7 +20,9 @@ public class SignalRBroadcaster : IChatBroadcaster
         _presenceTracker = presenceTracker;
     }
 
-    public Task SendMessageToChannelAsync(string channelName, MessageDto message)
+    // The exclusion only applies to the IRC gateway (SignalR clients render their own
+    // message from the broadcast echo), so the id is ignored here.
+    public Task SendMessageToChannelAsync(string channelName, MessageDto message, string? excludeConnectionId = null)
         => HubContext.Clients.Group(channelName).ReceiveMessage(message);
 
     public Task SendUserJoinedAsync(string channelName, string username, UserPresenceDto? presence, string? excludeConnectionId = null)