Constant FAILURE_REASON_MAX_LEN 

pub const FAILURE_REASON_MAX_LEN: usize = 512;

LOADBEARING: per-failure cap on failure_reason string length keeps the wire body bounded. Without truncation, runners can emit arbitrarily long stderr / response bodies that inflate the outbound queue’s JSON payloads and event-log row sizes. Runners pass their failure-reason strings through truncate_reason before constructing a RunnerOutcome::Fail.