Answer what the local client may do from the model

The Swing client menu decided on its own whether a group can be
(un)assigned and whether a client's channel can be joined, and the
server groups dialog borrowed the menu's check. These are rules about
the server, not about menus, so ServerModel answers them now and any
frontend asks it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ts3-client/core/src/test/java/com/ts3client/net/ServerModelRulesTest.java

@@ -0,0 +1,60 @@
+package com.ts3client.net;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class ServerModelRulesTest {
+
+    private final ServerModel model = new ServerModel();
+
+    private void grant(String permission, int value) {
+        model.putPermissionName(permission);
+        model.putSelfPermissionValue(permissionId(permission), value);
+    }
+
+    private int permissionId(String name) {
+        for (int id = 0; ; id++) {
+            if (name.equals(model.permissionName(id))) return id;
+        }
+    }
+
+    private static Group group(int addPower, int removePower) {
+        return new Group(9, "Group", 0, 0, 1, addPower, removePower);
+    }
+
+    @Test
+    void groupNeedsEnoughAddPowerAndRemovePowerToTakeAway() {
+        grant("i_group_needed_member_add_power", 50);
+        grant("i_group_needed_member_remove_power", 20);
+
+        assertTrue(model.canToggleGroup(group(50, 0), false));
+        assertFalse(model.canToggleGroup(group(51, 0), false));
+        assertTrue(model.canToggleGroup(group(50, 20), true));
+        assertFalse(model.canToggleGroup(group(50, 21), true));
+    }
+
+    @Test
+    void unlimitedPowerIsMinusOneOnEitherSide() {
+        grant("i_group_needed_member_add_power", 75);
+        grant("i_group_needed_member_remove_power", -1);
+
+        assertFalse(model.canToggleGroup(group(-1, 0), false), "a group needing unlimited power");
+        assertTrue(model.canToggleGroup(group(75, 1000), true), "unlimited remove power");
+    }
+
+    @Test
+    void joiningNeedsAVisibleChannelWithRoom() {
+        ChannelNode channel = model.putChannel(1, "Full", 0, 0);
+        model.putClient(5, "Someone", 1);
+        assertFalse(model.canJoinChannel(1), "not subscribed");
+
+        channel.subscribed = true;
+        assertTrue(model.canJoinChannel(1));
+        channel.maxClients = 1;
+        channel.clients.add(model.getClient(5));
+        assertFalse(model.canJoinChannel(1), "full");
+        assertFalse(model.canJoinChannel(2), "unknown");
+    }
+}