Extract counter command authorization into testable helpers

Pull the pure guard logic out of cmdDelCounter and cmdSetCounterActive
  into static evaluateDelCounter/evaluateSetCounterActive methods, leaving
  the handlers as thin shells over the decision. Add command_zone_tests
  covering every authorization branch of both evaluators.

tests/command_zone_tests/new_counter_id_test.cpp

@@ -0,0 +1,76 @@
+#include "../movecard_tests/server_test_helpers.h"
+#include "game/server_counter.h"
+#include "game/server_game.h"
+#include "game/server_player.h"
+#include "server_room.h"
+
+#include <gtest/gtest.h>
+#include <libcockatrice/protocol/pb/color.pb.h>
+#include <libcockatrice/protocol/pb/serverinfo_user.pb.h>
+#include <libcockatrice/rng/rng_abstract.h>
+#include <libcockatrice/utility/counter_ids.h>
+
+RNG_Abstract *rng = nullptr; // required by linked server code
+
+namespace
+{
+// Wires up a Server_Player backed by a minimal fake game so that counters can be
+// added directly via addCounter() to exercise newCounterId() in isolation.
+// newCounterId() depends only on the player's counter map, not on game state,
+// so the game never needs to be started.
+struct PlayerFixture
+{
+    ServerInfo_User user;
+    FakeServer server;
+    Server_Room room{0, 0, "", "", "", "", false, "", {}, &server};
+    Server_Game game{user,  1,     "",    "",    2,  QList<int>(), false, false,
+                     false, false, false, false, 20, false,        false, &room};
+    Server_Player player{&game, 1, user, false, nullptr};
+
+    ~PlayerFixture()
+    {
+        player.clearZones(); // owns and deletes any counters added during the test
+    }
+};
+} // namespace
+
+// With no counters at all, the first allocated id must be FirstUserId, never 0.
+TEST(NewCounterId, ReturnsFirstUserIdWhenNoCounters)
+{
+    PlayerFixture f;
+    EXPECT_EQ(f.player.newCounterId(), CounterIds::FirstUserId);
+}
+
+// Reserved ids 0-9 (standard player counters and the commander tax counters) must
+// not drag a new user counter down into the reserved range.
+TEST(NewCounterId, SkipsReservedRangeWhenOnlyReservedCountersExist)
+{
+    PlayerFixture f;
+    f.player.addCounter(new Server_Counter(3, "g", color(), 20, 0));
+    f.player.addCounter(new Server_Counter(CounterIds::PartnerTax, CounterNames::PartnerTax, color(), 20, 0));
+    EXPECT_EQ(f.player.newCounterId(), CounterIds::FirstUserId);
+}
+
+// Once user counters exist, the next id is one above the highest of them.
+TEST(NewCounterId, ReturnsNextIdAboveHighestUserCounter)
+{
+    PlayerFixture f;
+    f.player.addCounter(new Server_Counter(CounterIds::FirstUserId, "a", color(), 20, 0)); // 10
+    f.player.addCounter(new Server_Counter(15, "b", color(), 20, 0));
+    EXPECT_EQ(f.player.newCounterId(), 16);
+}
+
+// A reserved counter present alongside user counters must not affect the result.
+TEST(NewCounterId, IgnoresReservedCountersWhenUserCountersPresent)
+{
+    PlayerFixture f;
+    f.player.addCounter(new Server_Counter(5, "r", color(), 20, 0));     // reserved range
+    f.player.addCounter(new Server_Counter(11, "user", color(), 20, 0)); // user range
+    EXPECT_EQ(f.player.newCounterId(), 12);
+}
+
+int main(int argc, char **argv)
+{
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}