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/ftp_test.go

@@ -0,0 +1,33 @@
+//go:build plugin_ftp || !plugin_selective
+
+package services
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestClassifyFTPErrorType(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want ErrorType
+	}{
+		{"nil error", nil, ErrorTypeUnknown},
+		{"530 login incorrect", errors.New("530 login incorrect"), ErrorTypeAuth},
+		{"530 not logged in", errors.New("530 not logged in"), ErrorTypeAuth},
+		{"authentication failed", errors.New("authentication failed"), ErrorTypeAuth},
+		{"too many connections", errors.New("421 there are too many connections"), ErrorTypeNetwork},
+		{"connection refused", errors.New("connection refused"), ErrorTypeNetwork},
+		{"random error", errors.New("random error"), ErrorTypeUnknown},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := classifyFTPErrorType(tt.err)
+			if got != tt.want {
+				t.Errorf("classifyFTPErrorType(%v) = %v, want %v", tt.err, got, tt.want)
+			}
+		})
+	}
+}