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_reward_test.go

@@ -0,0 +1,126 @@
+package channelserver
+
+import (
+	"testing"
+
+	"erupe-ce/network/mhfpacket"
+)
+
+func TestHandleMsgMhfGetAdditionalBeatReward(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	pkt := &mhfpacket.MsgMhfGetAdditionalBeatReward{
+		AckHandle: 12345,
+	}
+
+	handleMsgMhfGetAdditionalBeatReward(session, pkt)
+
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No response packet queued")
+	}
+}
+
+func TestHandleMsgMhfGetUdRankingRewardList(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	pkt := &mhfpacket.MsgMhfGetUdRankingRewardList{
+		AckHandle: 12345,
+	}
+
+	handleMsgMhfGetUdRankingRewardList(session, pkt)
+
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No response packet queued")
+	}
+}
+
+func TestHandleMsgMhfGetRewardSong(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	pkt := &mhfpacket.MsgMhfGetRewardSong{
+		AckHandle: 12345,
+	}
+
+	handleMsgMhfGetRewardSong(session, pkt)
+
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No response packet queued")
+	}
+}
+
+func TestHandleMsgMhfUseRewardSong(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("handleMsgMhfUseRewardSong panicked: %v", r)
+		}
+	}()
+
+	handleMsgMhfUseRewardSong(session, nil)
+}
+
+func TestHandleMsgMhfAddRewardSongCount(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("handleMsgMhfAddRewardSongCount panicked: %v", r)
+		}
+	}()
+
+	handleMsgMhfAddRewardSongCount(session, nil)
+}
+
+func TestHandleMsgMhfAcquireMonthlyReward(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	pkt := &mhfpacket.MsgMhfAcquireMonthlyReward{
+		AckHandle: 12345,
+	}
+
+	handleMsgMhfAcquireMonthlyReward(session, pkt)
+
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No response packet queued")
+	}
+}
+
+func TestHandleMsgMhfAcceptReadReward(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("handleMsgMhfAcceptReadReward panicked: %v", r)
+		}
+	}()
+
+	handleMsgMhfAcceptReadReward(session, nil)
+}