feat(api): add request body decoding with Content-Encoding support

- Introduced `ReadRequestBody` helper function to support decoding request bodies based on "Content-Encoding" (e.g., `zstd`).
- Replaced `c.GetRawData()` with `ReadRequestBody` across handlers to enable decoding.
- Added test case to validate `zstd` decoding for compact responses.

sdk/api/handlers/request_body.go

@@ -0,0 +1,73 @@
+package handlers
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"github.com/klauspost/compress/zstd"
+)
+
+// ReadRequestBody reads the incoming request body and decodes supported
+// Content-Encoding values before handlers inspect JSON fields.
+func ReadRequestBody(c *gin.Context) ([]byte, error) {
+	raw, err := c.GetRawData()
+	if err != nil {
+		return nil, err
+	}
+
+	encoding := ""
+	if c != nil && c.Request != nil {
+		encoding = strings.TrimSpace(c.Request.Header.Get("Content-Encoding"))
+	}
+	if encoding == "" || strings.EqualFold(encoding, "identity") {
+		return raw, nil
+	}
+
+	decoded, err := decodeRequestBody(raw, encoding)
+	if err != nil {
+		if json.Valid(raw) {
+			return raw, nil
+		}
+		return nil, err
+	}
+	return decoded, nil
+}
+
+func decodeRequestBody(raw []byte, encoding string) ([]byte, error) {
+	parts := strings.Split(encoding, ",")
+	body := raw
+	for i := len(parts) - 1; i >= 0; i-- {
+		enc := strings.ToLower(strings.TrimSpace(parts[i]))
+		switch enc {
+		case "", "identity":
+			continue
+		case "zstd":
+			decoded, err := decodeZstdRequestBody(body)
+			if err != nil {
+				return nil, err
+			}
+			body = decoded
+		default:
+			return nil, fmt.Errorf("unsupported request content encoding: %s", enc)
+		}
+	}
+	return body, nil
+}
+
+func decodeZstdRequestBody(raw []byte) ([]byte, error) {
+	decoder, err := zstd.NewReader(bytes.NewReader(raw))
+	if err != nil {
+		return nil, fmt.Errorf("failed to create zstd request decoder: %w", err)
+	}
+	defer decoder.Close()
+
+	decoded, err := io.ReadAll(decoder)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decode zstd request body: %w", err)
+	}
+	return decoded, nil
+}