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/kafka_test.go

@@ -4,6 +4,7 @@ package services
 
 import (
 	"encoding/binary"
+	"errors"
 	"io"
 	"testing"
 )
@@ -61,3 +62,25 @@ func TestKafkaRecvRejectsShortResponse(t *testing.T) {
 		t.Fatal("kafkaRecv() error = nil, want invalid length error")
 	}
 }
+
+func TestClassifyKafkaErrorType(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want ErrorType
+	}{
+		{"nil", nil, ErrorTypeUnknown},
+		{"sasl auth failed", errors.New("sasl authentication failed"), ErrorTypeAuth},
+		{"unauthorized", errors.New("unauthorized"), ErrorTypeAuth},
+		{"broker not available", errors.New("broker not available"), ErrorTypeNetwork},
+		{"connection refused", errors.New("connection refused"), ErrorTypeNetwork},
+		{"unknown", errors.New("random kafka error"), ErrorTypeUnknown},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := classifyKafkaErrorType(tt.err); got != tt.want {
+				t.Errorf("classifyKafkaErrorType() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}