Avoid replaying generated image payloads

[cbusillo]

Summary

• stop converting prior image_generation_call outputs back into full data:image/... user images during stateless replay
• replace generated image payloads with a bounded assistant text placeholder that preserves status, byte count, and a capped revised prompt
• add a two-turn integration test proving the second request no longer resends generated image payload bytes

Why

Generated images were being recorded in history and replayed as full input images on later turns, which could burn rate limit and request budget repeatedly. This keeps useful context without resending the raw image data.

Tracking

Refs #68
Refs #63

Validation

• cargo test -p code-core image_generation -- --nocapture
• cargo test -p code-core --test image_history_replay generated_images_are_not_replayed_in_next_turn_request_payload -- --nocapture
• ./build-fast.sh