test: increase code coverage from 45.1% to 48.3%

Add unit tests across multiple packages:
- byteframe: SetBE/SetLE byte order switching
- config: Mode.String() for all safe version ranges
- mhfpacket: 28 Parse methods, 5 Build methods, empty packet builds,
  variable-length packets, NOT IMPLEMENTED error paths, UpdateWarehouse
- network: PacketID.String() for known IDs, out-of-range, and all valid
- channelserver: handleMsgMhfGetPaperData (6 switch cases), grpToGR
  (11 input values), gacha handlers, TimeGameAbsolute, equipSkinHistSize
  (4 config branches), guild mission handlers, dumpSaveData disabled path
- entranceserver: makeHeader with various inputs

network/packetid_string_test.go

@@ -0,0 +1,52 @@
+package network
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestPacketIDString_KnownIDs(t *testing.T) {
+	tests := []struct {
+		id   PacketID
+		want string
+	}{
+		{MSG_HEAD, "MSG_HEAD"},
+		{MSG_SYS_ACK, "MSG_SYS_ACK"},
+		{MSG_SYS_PING, "MSG_SYS_PING"},
+		{MSG_SYS_LOGIN, "MSG_SYS_LOGIN"},
+		{MSG_MHF_SAVEDATA, "MSG_MHF_SAVEDATA"},
+		{MSG_MHF_CREATE_GUILD, "MSG_MHF_CREATE_GUILD"},
+		{MSG_SYS_reserve1AF, "MSG_SYS_reserve1AF"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.want, func(t *testing.T) {
+			got := tt.id.String()
+			if got != tt.want {
+				t.Errorf("PacketID(%d).String() = %q, want %q", tt.id, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestPacketIDString_OutOfRange(t *testing.T) {
+	// An ID beyond the known range should return "PacketID(N)"
+	id := PacketID(9999)
+	got := id.String()
+	if !strings.HasPrefix(got, "PacketID(") {
+		t.Errorf("out-of-range PacketID String() = %q, want prefix 'PacketID('", got)
+	}
+}
+
+func TestPacketIDString_AllValid(t *testing.T) {
+	// Verify all valid PacketIDs produce non-empty strings
+	for i := PacketID(0); i <= MSG_SYS_reserve1AF; i++ {
+		got := i.String()
+		if got == "" {
+			t.Errorf("PacketID(%d).String() returned empty string", i)
+		}
+		if strings.HasPrefix(got, "PacketID(") {
+			t.Errorf("PacketID(%d).String() = %q, expected named constant", i, got)
+		}
+	}
+}