test: expand channelserver coverage from 20% to 25%

Add tests for:
- Discord handlers (getPlayerSlice, getCharacterList)
- House handlers (boxToBytes, HouseData, Title structs)
- Mail struct tests
- Mercenary handlers (Partner, HunterNavi structs)
- Shop/Gacha handlers (writeShopItems, ShopItem, Gacha structs)
- Additional handler coverage for guild, tower, and simple handlers
- Stage handler tests for binary operations and enumeration
- Channel server tests for BroadcastMHF and session management

server/channelserver/handlers_mercenary_test.go

@@ -0,0 +1,27 @@
+package channelserver
+
+import (
+	"testing"
+
+	"erupe-ce/network/mhfpacket"
+)
+
+func TestHandleMsgMhfLoadLegendDispatch(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	pkt := &mhfpacket.MsgMhfLoadLegendDispatch{
+		AckHandle: 12345,
+	}
+
+	handleMsgMhfLoadLegendDispatch(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")
+	}
+}