Handle image processing errors gracefully with automatic retry

[rossnelson]

Summary

• Adds graceful recovery when the API cannot process an attached image: strips image blocks from the prompt and re-enqueues the request so the user's text is still fulfilled
• Introduces IMAGE_ERROR failure type and retried_without_images recovery action for explicit image error classification
• Defers Slack error notifications to after failure classification, so users see a contextual warning (image dropped) instead of a raw error message

Test plan

• Send a request with a corrupt/unsupported image and verify the bot posts a warning then retries with text only
• Send an image-only request (no text) with a bad image and verify the bot posts an appropriate "no text content" error
• Send a normal text request and verify behavior is unchanged
• Trigger a non-image error (e.g. transient network failure) and verify the original error notification still appears