[codex] reject empty base64 image inputs

[rhan-oai]

Summary

• Map Responses API .image_url invalid_value 400s into InvalidImageRequest so existing invalid-image recovery can run for empty/invalid base64 image URL failures.
• Replace empty data:image/*;base64, user images with model-visible text instead of emitting an input_image item.
• Replace empty base64 image outputs from MCP and dynamic tool paths with text feedback before they are serialized as tool output images.

Why

Empty base64 image URLs are a known low-level poisoned-thread shape. They are always invalid for the Responses API, and letting them through causes later turns or resumes to replay the same bad image_url and fail before the model can recover.

This keeps the scope intentionally narrow: it does not add a broad image validator, durable history patching, or all-image replacement semantics.

Validation

• just fmt
• cargo test -p codex-api
• cargo test -p codex-protocol
• git diff --check

Not run: full workspace test suite.