On the openai-codex (Responses API) route a tool result serialized to a
string-only function_call_output, dropping ImageBlock content, and the
agent loop's tool-image mirror only fired for provider "openai". So
images returned by read reached the TUI but never the model, which then
correctly reported it received no image content.
Extend the mirror to fire for "openai-codex" too (its client already
serializes user-message images as input_image, so the bytes arrive),
and have the codex tool-result serializer emit a short placeholder for
an image-only result instead of an empty output the API may reject.
Adds a test covering both behaviors.
