fix: prevent infinite retry loop for images exceeding 5MB

- Change MAX_IMAGE_BYTES from 6MB to 5MB to match Anthropic API limit
- Add isImageSizeError() to detect image size errors from API
- Handle image size errors with user-friendly message instead of retry
- Prevent failover for image size errors (not retriable)

Fixes #2271

src/media/constants.ts

@@ -1,4 +1,4 @@
-export const MAX_IMAGE_BYTES = 6 * 1024 * 1024; // 6MB
+export const MAX_IMAGE_BYTES = 5 * 1024 * 1024; // 5MB (Anthropic API limit)
 export const MAX_AUDIO_BYTES = 16 * 1024 * 1024; // 16MB
 export const MAX_VIDEO_BYTES = 16 * 1024 * 1024; // 16MB
 export const MAX_DOCUMENT_BYTES = 100 * 1024 * 1024; // 100MB