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

src/EchoHub.Core/Contracts/IChatBroadcaster.cs

@@ -4,7 +4,14 @@ namespace EchoHub.Core.Contracts;
 
 public interface IChatBroadcaster
 {
-    Task SendMessageToChannelAsync(string channelName, MessageDto message);
+    /// <summary>
+    /// Broadcast a chat message to a channel. <paramref name="excludeConnectionId"/> is the
+    /// connection the message originated from (IRC convention: never echo a message back to
+    /// the connection that sent it — its client already displayed it locally). Other
+    /// connections of the same user (e.g. an IRC session alongside a TUI session) still
+    /// receive the message.
+    /// </summary>
+    Task SendMessageToChannelAsync(string channelName, MessageDto message, string? excludeConnectionId = null);
     Task SendUserJoinedAsync(string channelName, string username, UserPresenceDto? presence, string? excludeConnectionId = null);
     Task SendUserLeftAsync(string channelName, string username);
     Task SendChannelUpdatedAsync(ChannelDto channel, string? channelName = null);

src/EchoHub.Core/Contracts/IChatService.cs

@@ -13,8 +13,10 @@ public interface IChatService
     Task<(List<MessageDto> History, string? Error, bool PasswordRequired)> JoinChannelAsync(string connectionId, Guid userId, string username, string channelName, string? password = null);
     Task LeaveChannelAsync(string connectionId, string username, string channelName);
 
-    // Messaging
-    Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content);
+    // Messaging. originConnectionId identifies the connection the message came from so
+    // broadcasters can avoid echoing it back to that one connection (IRC convention);
+    // the sender's other sessions still receive it.
+    Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content, string? originConnectionId = null);
     Task<List<MessageDto>> GetChannelHistoryAsync(string channelName, int count, int offset = 0);
 
     // Presence