docs: add the rationale doc for AG2-015

[bradAGI]

Pairs with trustabl/trustabl-rules#101 (AG2-015) and trustabl/trustabl#138 (fixture + tests).

Why AG2-015 exists

Six packs ship an idempotency rule — CSDK-006/016, OAI-009/019, ADK-006, MCP-007, CREW-006, PYD-007. AutoGen ships none.

The argument the doc makes

A retry is not an unusual event in an agent — it is the normal response to an ambiguous result. That is at-least-once delivery with a language model as the retry policy, and the retry policy cannot inspect the remote's state.

AutoGen adds two retry paths a single-agent loop does not have, and both come from its conversation model rather than the tool:

1. Persistence. A tool response is a message re-sent in full on every later turn. An inconclusive call does not scroll out of reach — it stays in front of the model, with its arguments, for the rest of the run. The window for a duplicate is not the next turn; it is every turn.
2. A retry issued by a different agent. In a GroupChat the manager selects who speaks next, and any selected agent can re-issue the tool. It reads the same transcript but holds no private memory of having committed the side effect, so the duplicate is issued by a participant structurally unable to know it is a duplicate.

Confidence 0.55 is defended, not glossed

It is the lowest in this pack, and the doc says why in both directions: the rule reasons from a name and a parameter list. update_cache prefix-matches and is harmless; book_flight and transfer_funds carry no listed prefix and are never examined at all. The prefix list is a heuristic, not a taxonomy of side effects.

The exclusions lead with the non-fix

The most important limitation is the one that looks like a fix:

Whether the key is stable. A key derived from a fresh uuid4() at call time is regenerated on the retry, so it deduplicates nothing while looking exactly like a fix.

Also named: a declared key the body never forwards, a downstream service that ignores keys entirely, and deduplication done in a wrapper the signature check cannot see.

One recommendation specific to this framework

In a GroupChat, assume the retry may come from a different agent than the original caller. The key has to travel in the transcript, not in one agent's private state.

And: put "reuse the same idempotency_key when retrying" in the tool description, because the model chooses the argument — if it is not told, it invents a new one on the retry.

Gate

Two pre-existing errors remain on main (OAI-112, PYD-106), fixed by #53. AG2-015 is covered and consistent.