fix: add validation for empty media_item_id in bulk update

Add strict validation to return 400 Bad Request when any media_item_id
is empty in the bulk-update request, rather than treating it as a
partial failure with 200 OK.

This aligns the handler behavior with test expectations for the
BulkUpdateBooks_EmptyBookIDs test case.

internal/handlers/media.go

@@ -439,6 +439,12 @@ func (h *MediaHandler) HandleBulkUpdate(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "media_item_updates required"})
 	}
 
+	for _, update := range req.MediaItemUpdates {
+		if update.MediaItemID == "" {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "media_item_id cannot be empty"})
+		}
+	}
+
 	results := make([]map[string]interface{}, 0)
 	successCount := 0
 	failedCount := 0