feat(serialization): implement Gin JSON/YAML binding

- Introduce SubstituteEnvReader that replaces ${VAR} patterns with environment variable
  values, properly quoted for JSON/YAML compatibility
- Gin bindings (JSON/YAML) that use the environment-substituting reader
  for request body binding with validation support

internal/serialization/gin_binding_test.go

@@ -0,0 +1,50 @@
+package serialization_test
+
+import (
+	"bytes"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/yusing/godoxy/internal/serialization"
+	gperr "github.com/yusing/goutils/errs"
+)
+
+type TestStruct struct {
+	Value  string `json:"value"`
+	Value2 int    `json:"value2"`
+}
+
+func (t *TestStruct) Validate() gperr.Error {
+	if t.Value == "" {
+		return gperr.New("value is required")
+	}
+	if t.Value2 != 0 && (t.Value2 < 5 || t.Value2 > 10) {
+		return gperr.New("value2 must be between 5 and 10")
+	}
+	return nil
+}
+
+func TestGinBinding(t *testing.T) {
+
+	tests := []struct {
+		name    string
+		input   string
+		wantErr bool
+	}{
+		{"valid1", `{"value": "test", "value2": 7}`, false},
+		{"valid2", `{"value": "test"}`, false},
+		{"invalid1", `{"value2": 7}`, true},
+		{"invalid2", `{"value": "test", "value2": 3}`, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var dst TestStruct
+			body := bytes.NewBufferString(tt.input)
+			req := httptest.NewRequest("POST", "/", body)
+			err := serialization.GinJSONBinding{}.Bind(req, &dst)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("%s: Bind() error = %v, wantErr %v", tt.name, err, tt.wantErr)
+			}
+		})
+	}
+}