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/openai/openai_images_handlers.go

@@ -204,7 +204,7 @@ func (h *OpenAIAPIHandler) ImagesGenerations(c *gin.Context) {
 		return
 	}
 
-	rawJSON, err := c.GetRawData()
+	rawJSON, err := handlers.ReadRequestBody(c)
 	if err != nil {
 		c.JSON(http.StatusBadRequest, handlers.ErrorResponse{
 			Error: handlers.ErrorDetail{
@@ -435,7 +435,7 @@ func (h *OpenAIAPIHandler) imagesEditsFromMultipart(c *gin.Context) {
 }
 
 func (h *OpenAIAPIHandler) imagesEditsFromJSON(c *gin.Context) {
-	rawJSON, err := c.GetRawData()
+	rawJSON, err := handlers.ReadRequestBody(c)
 	if err != nil {
 		c.JSON(http.StatusBadRequest, handlers.ErrorResponse{
 			Error: handlers.ErrorDetail{