feat(simplex): treat an edited inbound message as a correction that supersedes the queued/in-flight original - #97317

Summary

Implements edited-message supersede for the SimpleX gateway adapter, closing #35535.

Today the SimpleX adapter handles only newChatItems; a chatItemUpdated event (the terminal-API notification emitted when a user edits a sent message) falls into the unhandled-event log, so the agent acts on the original — mistaken — text. This PR treats an inbound edit as "I meant this instead", correlated to the specific original message by platform message id:

• Original still queued (primary pending slot or overflow FIFO) → the queued MessageEvent's text is replaced in place. FIFO order, identity, and metadata are preserved; the correction is silent (no busy-ack).
• Original in-flight (the turn's originating message id, now recorded on TurnState.active_message_id) → the running agent is redirected with the corrected text, framed as [User edited their earlier message. Corrected message: "..."]. Falls back to steer() when the agent doesn't support active-turn redirect, and to normal queueing when neither primitive is available. Neither primitive cancels in-flight tools — that is the best available behavior without mutating the transcript (see design notes).
• Uncorrelated edits (no queued or in-flight match) → dropped with an info log rather than dispatched as a new message. Rationale in Open questions below.

Changes

plugins/platforms/simplex/adapter.py

• _handle_chat_item now extracts meta.itemId and sets message_id=str(itemId) on every inbound MessageEvent (the prerequisite correlation key — previously never populated).
• New chatItemUpdated branch routes the event's AChatItem through the same parse path with is_edit=True, tagging metadata={"is_edit": True}. The existing outgoing-direction guard means edits of the bot's own messages are ignored.

gateway/session_state.py

• TurnState.active_message_id records the message id of the event that started the turn; cleared by the existing TurnState.clear() at every turn boundary (no new lifecycle to maintain, no leak path).

gateway/run.py

• _replace_queued_message(): first-match, in-place text replacement across both queue levels, modeled on the existing _clear_goal_pending_continuations pattern.
• _handle_edit_supersede(): the correlation decision — queued replace → in-flight redirect/steer → drop+log. Fully synchronous (no await between the queue check, the in-flight check, and the decision), so there is no interleaving window with queue promotion. Inserted in _handle_message after authorization, ignored-channel, and plugin-hook guards; adapter producers other than SimpleX get the same behavior free by tagging metadata["is_edit"] + message_id.

Design notes

• Why not rewrite the transcript: the stale original text stays in history; per the project's prompt-caching constraint, past context is never mutated. The correction reaches the live turn via redirect/steer, mirroring existing mid-turn steering semantics.
• Why gateway-core, not adapter-only: correlating an edit to the specific queued/in-flight original requires the queue/turn structures the adapter can't see — as the issue describes. Any future adapter that tags edits (Signal and Discord both surface them today in other forms) inherits the behavior.
• Startup-resume path: the synthetic resume event carries no message id; its active_message_id stays None, so edits during a resumed turn fall to the uncorrelated policy rather than misfiring.

Open questions

• Uncorrelated-edit policy: dropping (with a log) rather than queueing as a new message is a judgment call — Pro-side reasoning in the design review was that an isolated out-of-context correction ("the") injected into a fresh turn confuses the model; the counter-argument is that a drop silently loses user intent. If maintainers prefer "queue uncorrelated edits as normal messages", it's a two-line change in _handle_edit_supersede.

Tests

• tests/gateway/test_simplex_plugin.py: 7 new tests — direct/group edit correlation, outgoing-direction suppression, message_id population on the normal path, image-content edits, missing-itemId fallback.
• tests/gateway/test_edit_supersede.py (new): 15 tests — primary-slot replace, overflow-FIFO replace, in-flight redirect, steer fallback, sentinel-window handling, stale-correlation drop, uncorrelated drop, non-edit pass-through.
• All 38 pass locally; the full tests/gateway/ suite shows only pre-existing environment flakes present on clean main.

Closes #35535