v0.5.1 — leaner checkpoints for structured agents

Agents that declare a response schema no longer checkpoint the text form of their response alongside structured — it was the same JSON stored twice, doubling the checkpoint size for no information.

Adjust if needed: code reading steps.{id}.text from a structured agent should read steps.{id}.structured (or a key within it) instead. Plain agents keep steps.{id}.text exactly as before.