feat(autogen): add AG2-015, mutating tool has no idempotency key

[bradAGI]

Paired with the engine PR on a branch of the same name, so rules-sync resolves this pack. Neither should merge alone.

The gap

Six packs ship an idempotency rule. AutoGen does not:

pack	rule		pack	rule
Claude Agent SDK	CSDK-006 / CSDK-016		MCP	MCP-007
OpenAI Agents SDK	OAI-009 / OAI-019		CrewAI	CREW-006
Google ADK	ADK-006		Pydantic AI	PYD-007
			AutoGen	— none —

AG2-015 uses the same name_has_prefix + param_name_matches predicate pair as CREW-006, at the same medium / 0.55.

Why it matters specifically here

The generic argument is that retries re-run side effects. AutoGen supplies more ways for the retry to happen than a single-agent loop does, and both come from its conversation model:

• The tool response is a message in a conversation that is re-sent in full on every later turn. A call whose result read as inconclusive does not scroll away — it stays visible and re-invitable for the rest of the run.
• In a GroupChat, any speaker the manager selects can re-issue it — including an agent that was not the original caller, and therefore cannot know the side effect already committed.

So the window in which a duplicate refund_payment can fire is the whole run, and the agent that fires it may be one with no memory of the first attempt.

The fix text names the non-fix

Worth flagging, because it is the common way this gets "fixed" wrongly:

Derive the key from the request's own identity, not from a fresh uuid4() per call — a key regenerated on the retry deduplicates nothing.

A parameter named idempotency_key filled with a fresh UUID at each call satisfies the rule while providing no deduplication at all. The rule cannot see this; the text says so.

Verified end to end

AG2-015 [medium] refund_payment tools.py:6

The sample's second tool is the same refund_payment with an idempotency_key parameter threaded through to the backing call; the rule did not fire on it.