feat: make ServerConnection accessible in ServerLoginPluginMessageEvent (minekube#664)

Add serverConn field and ServerConnection() getter to
ServerLoginPluginMessageEvent, following the same pattern used by
ServerResourcePackSendEvent. This allows event subscribers to identify
which server connection triggered the login plugin message.

Closes minekube#663

https://claude.ai/code/session_01DkvnojAjJ8KS2jCPq27rWe

Co-authored-by: Claude <noreply@anthropic.com>

Author: robinbraemer

pkg/edition/java/proxy/events.go

@@ -1089,6 +1089,7 @@ type ServerLoginPluginMessageEvent struct {
 	id         message.ChannelIdentifier
 	contents   []byte
 	sequenceID int
+	serverConn *serverConnection
 
 	result ServerLoginPluginMessageResult
 }
@@ -1103,6 +1104,11 @@ func (e *ServerLoginPluginMessageEvent) SequenceID() int {
 	return e.sequenceID
 }
 
+// ServerConnection returns the associated server connection.
+func (e *ServerLoginPluginMessageEvent) ServerConnection() ServerConnection {
+	return e.serverConn
+}
+
 func (e *ServerLoginPluginMessageEvent) Result() *ServerLoginPluginMessageResult {
 	return &e.result
 }

pkg/edition/java/proxy/session_backend_login.go

@@ -160,6 +160,7 @@ func (b *backendLoginSessionHandler) handleLoginPluginMessage(p *packet.LoginPlu
 			id:         identifier,
 			contents:   p.Data,
 			sequenceID: p.ID,
+			serverConn: b.serverConn,
 		}
 		b.eventMgr.Fire(e)
 		if e.Result().Allowed() {