feat: placeholder for image that can't be decoded to prevent 400 (#6773)

codex-rs/utils/image/src/error.rs

@@ -1,3 +1,4 @@
+use image::ImageError;
 use image::ImageFormat;
 use std::path::PathBuf;
 use thiserror::Error;
@@ -23,3 +24,15 @@ pub enum ImageProcessingError {
         source: image::ImageError,
     },
 }
+
+impl ImageProcessingError {
+    pub fn is_invalid_image(&self) -> bool {
+        matches!(
+            self,
+            ImageProcessingError::Decode {
+                source: ImageError::Decoding(_),
+                ..
+            }
+        )
+    }
+}