`stringifyNonErrorCause` is typed `string`, but its `try` returned
`JSON.stringify(value)`, which is `undefined` for functions, symbols, and
undefined causes — leaking undefined to callers that format nested ACP runtime
failures and expect a string. Fall back to a tag string when stringify yields
undefined, matching the already-correct sibling at `src/infra/errors.ts`.
Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Persistent ACP threads died on the second turn for Kiro: when the backend
can no longer resume a stale session, acpx raises a SessionResumeRequiredError
whose reason text varies by backend ("Resource not found" for Claude,
"Internal error" / RequestError -32603 for Kiro). The recovery gate matched
the human reason text and required "resource not found", so Kiro's "Internal
error" never triggered the fresh-session retry and the thread produced no
reply (ACP_TURN_FAILED).
Recover by acpx's structured detail code instead of the reason text: acpx
tags every such failure with detailCode "SESSION_RESUME_REQUIRED"
(retryable), independent of wording. The two AcpRuntimeError construction
seams were discarding detailCode, so preserve it on AcpRuntimeError and match
it across the error and its cause chain. This fixes every backend's
resume-required failure and is more precise than the reason regex — a generic
"Internal error" without the code is still surfaced rather than silently
retried.
Fixes#87830. Reported by @chouzz.
Adds broad inline comments and JSDoc for CLI, cron, outbound/channel, plugin SDK, ACP, shared helpers, net policy, and related utility contracts. Proof: git diff --check on latest exact head plus focused cron tests passed; CI had no failing checks observed before merge attempt.
