test: 补充单元测试覆盖率 29.9% → 36.6%

新建 18 个测试文件，追加 30 个已有测试文件，覆盖协议解析、
错误分类、CEL 表达式求值、YAML 反序列化、字节编码等纯函数。

plugins/services/snmp_test.go

@@ -0,0 +1,156 @@
+//go:build plugin_snmp || !plugin_selective
+
+package services
+
+import (
+	"encoding/asn1"
+	"testing"
+
+	"github.com/shadow1ng/fscan/common"
+)
+
+// --- buildSNMPGetRequest ---
+
+func TestBuildSNMPGetRequest(t *testing.T) {
+	oid := []int{1, 3, 6, 1, 2, 1, 1, 1, 0}
+
+	t.Run("returns non-empty bytes starting with ASN.1 SEQUENCE", func(t *testing.T) {
+		pkt := buildSNMPGetRequest("public", oid)
+		if len(pkt) == 0 {
+			t.Fatal("expected non-empty packet")
+		}
+		if pkt[0] != 0x30 {
+			t.Errorf("first byte = 0x%02x, want 0x30 (ASN.1 SEQUENCE)", pkt[0])
+		}
+	})
+
+	t.Run("different communities produce different lengths", func(t *testing.T) {
+		pkt1 := buildSNMPGetRequest("public", oid)
+		pkt2 := buildSNMPGetRequest("longercommunitystringhere", oid)
+		if len(pkt1) >= len(pkt2) {
+			t.Errorf("expected longer community to produce longer packet: len(public)=%d len(long)=%d", len(pkt1), len(pkt2))
+		}
+	})
+}
+
+// --- marshalOIDWithNull ---
+
+func TestMarshalOIDWithNull(t *testing.T) {
+	oid := []int{1, 3, 6, 1, 2, 1, 1, 1, 0}
+	result := marshalOIDWithNull(oid)
+
+	if len(result) == 0 {
+		t.Fatal("expected non-empty bytes")
+	}
+
+	// 应包含 OID tag (0x06) 和 NULL tag (0x05)
+	foundOID := false
+	foundNull := false
+	for _, b := range result {
+		if b == 0x06 {
+			foundOID = true
+		}
+		if b == 0x05 {
+			foundNull = true
+		}
+	}
+	if !foundOID {
+		t.Error("expected OID tag 0x06 in output")
+	}
+	if !foundNull {
+		t.Error("expected NULL tag 0x05 in output")
+	}
+}
+
+// --- parseSNMPResponse ---
+
+// buildTestSNMPResponse 构造最小合法 SNMPv2c GetResponse 包含 OctetString value
+func buildTestSNMPResponse(community string, value string) []byte {
+	valBytes, _ := asn1.Marshal(asn1.RawValue{Class: 0, Tag: 4, Bytes: []byte(value)})
+	oidBytes, _ := asn1.Marshal(asn1.ObjectIdentifier{1, 3, 6, 1, 2, 1, 1, 1, 0})
+
+	var vbContent []byte
+	vbContent = append(vbContent, oidBytes...)
+	vbContent = append(vbContent, valBytes...)
+	varbind, _ := asn1.Marshal(asn1.RawValue{Class: 0, Tag: 16, IsCompound: true, Bytes: vbContent})
+	varbindList, _ := asn1.Marshal(asn1.RawValue{Class: 0, Tag: 16, IsCompound: true, Bytes: varbind})
+
+	reqID, _ := asn1.Marshal(12345)
+	errStatus, _ := asn1.Marshal(0)
+	errIndex, _ := asn1.Marshal(0)
+
+	var pduContent []byte
+	pduContent = append(pduContent, reqID...)
+	pduContent = append(pduContent, errStatus...)
+	pduContent = append(pduContent, errIndex...)
+	pduContent = append(pduContent, varbindList...)
+
+	// GetResponse PDU: context-specific tag 2
+	pdu, _ := asn1.Marshal(asn1.RawValue{Class: 2, Tag: 2, IsCompound: true, Bytes: pduContent})
+
+	version, _ := asn1.Marshal(1) // SNMPv2c
+	comm, _ := asn1.Marshal([]byte(community))
+
+	var msgContent []byte
+	msgContent = append(msgContent, version...)
+	msgContent = append(msgContent, comm...)
+	msgContent = append(msgContent, pdu...)
+
+	msg, _ := asn1.Marshal(asn1.RawValue{Class: 0, Tag: 16, IsCompound: true, Bytes: msgContent})
+	return msg
+}
+
+func TestParseSNMPResponse(t *testing.T) {
+	t.Run("empty data returns empty", func(t *testing.T) {
+		got := parseSNMPResponse([]byte{})
+		if got != "" {
+			t.Errorf("got %q, want empty", got)
+		}
+	})
+
+	t.Run("invalid ASN.1 returns empty", func(t *testing.T) {
+		got := parseSNMPResponse([]byte{0xFF, 0xFF, 0xFF})
+		if got != "" {
+			t.Errorf("got %q, want empty", got)
+		}
+	})
+
+	t.Run("valid response returns sysDescr value", func(t *testing.T) {
+		want := "Linux router 5.4.0"
+		pkt := buildTestSNMPResponse("public", want)
+		got := parseSNMPResponse(pkt)
+		if got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	})
+}
+
+// --- buildCommunityList ---
+
+func TestBuildCommunityList(t *testing.T) {
+	p := NewSNMPPlugin()
+	cfg := &common.Config{}
+
+	list := p.buildCommunityList(cfg)
+
+	if len(list) == 0 {
+		t.Fatal("community list must not be empty")
+	}
+
+	hasPublic := false
+	hasPrivate := false
+	for _, c := range list {
+		if c == "public" {
+			hasPublic = true
+		}
+		if c == "private" {
+			hasPrivate = true
+		}
+	}
+	if !hasPublic {
+		t.Error("community list must contain 'public'")
+	}
+	if !hasPrivate {
+		t.Error("community list must contain 'private'")
+	}
+}