Fix nil interface trap in GetByNumber

Map lookup returns typed nil *Session for missing keys, which wraps into CallSession interface as non-nil typed nil. Nil check silently passes, causing nil dereference.

internal/server/server.go

@@ -357,7 +357,11 @@ func (s *Server) Unregister(number string) {
 func (s *Server) GetByNumber(number string) call.CallSession {
 	s.sessionsMu.RLock()
 	defer s.sessionsMu.RUnlock()
-	return s.sessions[number]
+	sess := s.sessions[number]
+	if sess == nil {
+		return nil
+	}
+	return sess
 }
 
 func (s *Server) GetAll() []call.CallSession {