Best-effort telemetry needs unconditional descriptor cleanup

Best-effort telemetry must release every parent-side file descriptor in an unconditional final cleanup path. Creating a nonblocking telemetry pipe is not enough: cancellation before reader attachment can combine with a failing child-stop operation and skip the normal close. Repeated occurrences then leak per-call resources even though telemetry was intended to be harmless. Test real descriptors and process boundaries, including spawn failure, cancel-before-return, and stop failure; verify SDK collection separately from a mocked callback. This applies to optional subprocess instrumentation with separate control and telemetry channels. It does not prove delivery for a child crash before event emission or for loss of the stable parent.