fix(tui): recover from stale forms - #44507

AI code review — automated review for reference, author can ignore or act on any point.

Solid fix for a real UX dead-end. The mechanism checks out end-to-end: both form endpoints declare declaredStatuses: [404, 409, 400, 401] (packages/client/src/promise/generated/client.ts:1331, :1343), so the promise client throws the parsed tagged body directly and the _tag-narrowing guards work as written. The early releaseMode + visible={!dismissed()} approach keeps cleanup ordering safe (mode stack splice by symbol id, layers inert once the mode pops), and the new regression tests cover both entry points.

Suggestions:

1. FormAlreadySettledError (409) still traps the user — packages/tui/src/routes/session/form.tsx:273-279 (reply) and :478-484 (cancel) only special-case FormNotFoundError. A form answered or cancelled by another client (TUI + web open concurrently — likely the most common staleness case) returns 409 {_tag: "FormAlreadySettledError"}, which falls through to setStore("error", ...) and leaves the prompt up even though server state has settled. Suggested fix: also import isFormAlreadySettledError (exported from @opencode-ai/client via generated/types.js) and route that case through dismissStale() as well — optionally after toast.info("Form was answered elsewhere") so users aren't confused by the silent disappearance. Keeping 400 inline is right since invalid-answer errors are user-fixable.

2. Idempotency guard in dismissStale — form.tsx:487-493. Two rapid Enter presses can put two replies in flight; both 404 and each runs invalidate + sync. A cheap if (dismissed()) return at the top avoids redundant refetch churn.

3. Custom handler flows — dismissal now also triggers when embedder-provided props.onReply/props.onCancel reject with FormNotFoundError (form.tsx:266-267, :471-472), releasing FORM_MODE and re-syncing the shared form store under a host that may own its own lifecycle. Probably desired for consistency; just confirming it's intentional.

Minor: nice touch that the test fixture already stubs /api/session/{id}/form GET (packages/tui/test/fixture/tui-client.ts:145), so the post-dismissal sync() resolves against an empty list rather than relying on the .catch(toast.error).

Superseded by #44516, which handles both missing and already-settled forms and restores the composer from local projection state before background revalidation.