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。

webscan/lib/eval_misc_test.go

@@ -0,0 +1,53 @@
+package lib
+
+import (
+	"testing"
+	"unicode"
+
+	"github.com/google/cel-go/common/types"
+)
+
+func TestRegisterMiscImplementations_TongdaDate(t *testing.T) {
+	overloads := registerMiscImplementations()
+
+	idx := make(map[string]int, len(overloads))
+	for i, o := range overloads {
+		idx[o.Operator] = i
+	}
+
+	i, ok := idx["tongda_date"]
+	if !ok {
+		t.Fatal("overload tongda_date not found")
+	}
+	fn := overloads[i].Function
+	if fn == nil {
+		t.Fatal("tongda_date Function field is nil")
+	}
+
+	result := fn()
+
+	if types.IsError(result) {
+		t.Fatalf("unexpected error: %v", result)
+	}
+
+	got, ok := result.(types.String)
+	if !ok {
+		t.Fatalf("expected types.String, got %T", result)
+	}
+
+	s := string(got)
+
+	t.Run("length_is_4", func(t *testing.T) {
+		if len(s) != 4 {
+			t.Errorf("tongda_date returned %q, want 4-char string", s)
+		}
+	})
+
+	t.Run("all_digits", func(t *testing.T) {
+		for _, r := range s {
+			if !unicode.IsDigit(r) {
+				t.Errorf("tongda_date returned %q, contains non-digit char %q", s, r)
+			}
+		}
+	})
+}