Dynamic prompt is not updated for each `agent_model.run`

What was going on
Issue pydantic/pydantic-ai#839 (closed, 21 comments): Hi there, Follow up on my slack question. The `dynamic prompt` is very sweet to provide up-to-date context to the LLM. However inside a single run many steps can occur, with the ability to mutate the state retained in `ctx.deps` (through tool calls). At the moment `dynamic prompt` is `updated` at the very begining of each run and then is `static` for the whole `run`. **I would like to request an option to have more fine-grained update for** `dynamic prompt` so it can `refresh` for each `agent_model.request()`.

The fix
If you need the prompt refreshed between steps inside a single run, use @agent.instructions instead of a dynamic prompt. Instructions are re-evaluated between tool-call rounds (since PR #1503, v0.1.1), which covers the per-step refresh the issue asked for.