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/agents/pi-embedded-helpers/errors.ts

@@ -467,6 +467,12 @@ export function isImageDimensionErrorMessage(raw: string): boolean {
   return Boolean(parseImageDimensionError(raw));
 }
 
+export function isImageSizeError(errorMessage?: string): boolean {
+  if (!errorMessage) return false;
+  const lower = errorMessage.toLowerCase();
+  return lower.includes("image exceeds") && lower.includes("mb");
+}
+
 export function isCloudCodeAssistFormatError(raw: string): boolean {
   return !isImageDimensionErrorMessage(raw) && matchesErrorPatterns(raw, ERROR_PATTERNS.format);
 }
@@ -478,6 +484,7 @@ export function isAuthAssistantError(msg: AssistantMessage | undefined): boolean
 
 export function classifyFailoverReason(raw: string): FailoverReason | null {
   if (isImageDimensionErrorMessage(raw)) return null;
+  if (isImageSizeError(raw)) return null;
   if (isRateLimitErrorMessage(raw)) return "rate_limit";
   if (isOverloadedErrorMessage(raw)) return "rate_limit";
   if (isCloudCodeAssistFormatError(raw)) return "format";