ReActV2: replay a stored answer turn under finish: :submit and :tools

[deepfates]

What was wrong

A history turn a host stores with the task's inputs and its one output, and no step of the loop's own, is meant to replay as the model's message holding that output (CHANGELOG 0.6.0: "a stored turn that carries only the task's answer is replayed as that answer"). ReActV2 does this in step_turn/2, which reads such a turn as a step that answered in text (next_thought = the answer, no calls) before any adapter formats it.

Since #343 that reading was gated on text_output(signature, finish), which is :submit for every program with finish: :submit or finish: :tools. So under those endings the turn was passed through untouched, and the adapter rendered only the step outputs it found (next_thought, tool_calls), of which the turn has none. The model's request carried the turn's inputs as a user message and then nothing: no assistant message, under both history_replay: :trajectory and :actions.

Request for a history of one stored turn {ticket: "Recap notice", reply: "THE RECAP TEXT"}, then a new ticket, before this change:

finish	replay	messages after system
auto	trajectory / actions	user "Recap notice", assistant "THE RECAP TEXT", user "New event"
submit	trajectory / actions	user "Recap notice", user "New event"
tools	trajectory / actions	user "Recap notice", user "New event"

Turns the loop writes itself are not affected: a plain-text answer step stores next_thought beside the output, and a submit answer stores the submit call and its result, so both already replay.

The change

step_turn/2 reads a stored answer turn the same way under every ending: it requires a one-output signature and a string value, not a plain-text ending. The turn carries the task's output, so the Chat adapter treats it as an answer, never as a thought: no "not sent" line follows it, the same as a plain-text loop's answer step replayed under submit (test/react_v2_ending_switch_test.exs). The history the caller holds is unchanged.

What changes in requests: under finish: :submit and :tools, one assistant message holding the stored output now follows the stored turn's inputs. Nothing else in any request changes. Under finish: :auto, a one-output signature whose output is constrained (an enum, a pattern) or not typed string, which ends through submit, gets the same replay when the stored value is a string. It too showed only the inputs before.

Tests

test/react_v2_host_answer_turn_test.exs: for each of finish: :auto | :submit | :tools × history_replay: :trajectory | :actions, the request after a stored answer turn is user (inputs), assistant (the answer), user (new inputs). Four of the six fail without the change: submit and tools, both replays. One more test checks that the returned history keeps the turn as it was written.

mix check: 3728 tests, 0 failures.

A patch release: no option and no public API is added.