test: expand channelserver coverage from 16% to 20%

Add comprehensive tests for handler functions and utilities:
- Achievement system (GetAchData, curves, mappings)
- Language system (getLangStrings)
- Core handlers (empty handlers, simple responses)
- Cafe/boost handlers
- Diva defense handlers
- Reward, caravan, tactics handlers
- Festa/ranking handlers
- Cast binary constants

server/channelserver/handlers_cast_binary_test.go

@@ -0,0 +1,80 @@
+package channelserver
+
+import (
+	"testing"
+)
+
+func TestBinaryMessageTypeConstants(t *testing.T) {
+	tests := []struct {
+		name     string
+		constant int
+		expected int
+	}{
+		{"BinaryMessageTypeState", BinaryMessageTypeState, 0},
+		{"BinaryMessageTypeChat", BinaryMessageTypeChat, 1},
+		{"BinaryMessageTypeQuest", BinaryMessageTypeQuest, 2},
+		{"BinaryMessageTypeData", BinaryMessageTypeData, 3},
+		{"BinaryMessageTypeMailNotify", BinaryMessageTypeMailNotify, 4},
+		{"BinaryMessageTypeEmote", BinaryMessageTypeEmote, 6},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.constant != tt.expected {
+				t.Errorf("%s = %d, want %d", tt.name, tt.constant, tt.expected)
+			}
+		})
+	}
+}
+
+func TestBroadcastTypeConstants(t *testing.T) {
+	tests := []struct {
+		name     string
+		constant int
+		expected int
+	}{
+		{"BroadcastTypeTargeted", BroadcastTypeTargeted, 0x01},
+		{"BroadcastTypeStage", BroadcastTypeStage, 0x03},
+		{"BroadcastTypeServer", BroadcastTypeServer, 0x06},
+		{"BroadcastTypeWorld", BroadcastTypeWorld, 0x0a},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.constant != tt.expected {
+				t.Errorf("%s = %d, want %d", tt.name, tt.constant, tt.expected)
+			}
+		})
+	}
+}
+
+func TestCommandsMapInitialized(t *testing.T) {
+	// commands map should be initialized by init()
+	if commands == nil {
+		t.Error("commands map should be initialized")
+	}
+}
+
+func TestSendServerChatMessage(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	// Should not panic
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("sendServerChatMessage panicked: %v", r)
+		}
+	}()
+
+	sendServerChatMessage(session, "Test message")
+
+	// Should queue a packet
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No packet queued")
+	}
+}