test: add comprehensive tests for critical components

Add tests for previously untested critical areas:

- QueueServerSelectorTest: Tests all load balancing strategies
  (ROUND_ROBIN, LEAST_PLAYERS, RANDOM), edge cases like empty
  servers, offline servers, and strategy changes

- ConfigMigratorTest: Tests V2 to V3 migration including
  placeholder updates, queueServer to queueServers conversion,
  and idempotent migration behavior

- ConfigTest: Tests getQueueType permission matching, queue type
  ordering, findGroupByTarget case sensitivity, kickWhenDownServers
  placeholder expansion, and queue group configuration

- MainCommandSharedTest: Tests command parsing, shadowban duration
  parsing (d/h/m/s), permission checks, tab completion, and
  documents the NumberFormatException bug with invalid durations

Also adds missing Config setters for testing:
- setServerName, setQueueLoadBalancing, setRawKickWhenDownServers

Author: AlexProgrammerDE

shared/src/main/java/net/pistonmaster/pistonqueue/shared/config/Config.java

@@ -595,6 +595,18 @@ public void setUsernameRegexMessage(String message) {
     this.usernameRegexMessage = message;
   }
 
+  public void setServerName(String serverName) {
+    this.serverName = serverName;
+  }
+
+  public void setQueueLoadBalancing(LoadBalancingStrategy strategy) {
+    this.queueLoadBalancing = strategy;
+  }
+
+  public void setRawKickWhenDownServers(List<String> servers) {
+    this.rawKickWhenDownServers = new ArrayList<>(servers);
+  }
+
   private void rebuildKickWhenDownServers() {
     List<String> resolved = new ArrayList<>();
     for (String text : rawKickWhenDownServers) {