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

server/entranceserver/make_resp_extended_test.go

@@ -0,0 +1,35 @@
+package entranceserver
+
+import (
+	"testing"
+)
+
+// TestMakeHeader tests the makeHeader function with various inputs
+func TestMakeHeader(t *testing.T) {
+	tests := []struct {
+		name       string
+		data       []byte
+		respType   string
+		entryCount uint16
+		key        byte
+	}{
+		{"empty data", []byte{}, "SV2", 0, 0x00},
+		{"small data", []byte{0x01, 0x02, 0x03}, "SV2", 1, 0x00},
+		{"SVR type", []byte{0xAA, 0xBB}, "SVR", 2, 0x42},
+		{"USR type", []byte{0x01}, "USR", 1, 0x00},
+		{"larger data", make([]byte, 100), "SV2", 5, 0xFF},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := makeHeader(tt.data, tt.respType, tt.entryCount, tt.key)
+			if len(result) == 0 {
+				t.Error("makeHeader returned empty result")
+			}
+			// First byte should be the key
+			if result[0] != tt.key {
+				t.Errorf("first byte = %x, want %x", result[0], tt.key)
+			}
+		})
+	}
+}