sanitize invalid image history

[rhan-oai]

Summary

• map Responses invalid-image failures, including URL download failures, into InvalidImageRequest
• replace bad user/tool images in conversation history with model-visible feedback instead of rolling back turns
• let tool-output image failures continue sampling while user-supplied image failures surface a BadRequest error

Validation

• cargo test -p codex-api api_bridge
• cargo test -p codex-core replace_newest_images
• cargo test -p codex-core invalid_user_image_is_sanitized_before_follow_up_turn
• cargo test -p codex-core invalid_tool_image_is_replaced_and_returned_to_model
• git diff --check