Add: UserFactory to create new users and resolve name conflicts

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/UserFactory.java

@@ -0,0 +1,28 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionId;
+
+public class UserFactory {
+    private final UserRegistry registry;
+
+    public UserFactory(UserRegistry registry) {
+        this.registry = registry;
+    }
+
+    public User create(String desiredName, SessionId sessionId) {
+        var result = registry.registerIfAvailable(desiredName, sessionId);
+        if (result.isPresent()) {
+            return result.get();
+        }
+
+        int suffix = 1;
+        while (true) {
+            String candidate = desiredName + "_" + String.format("%03d", suffix);
+            result = registry.registerIfAvailable(candidate, sessionId);
+            if (result.isPresent()) {
+                return result.get();
+            }
+            suffix++;
+        }
+    }
+}