Cross-namespace schedule tampering via unbound completion callbacks

Authorization bypass in Temporal Server allows remote authenticated users to alter Scheduler V2 schedules via completion callbacks. The History service delivers caller-supplied internal callbacks to the namespace named in their token without binding it to the originating namespace or authorizing access there. Exploitation requires namespace write access and target identifiers plus running-action timing in an existing Scheduler V2 schedule; this off-by-default feature permits overlap-policy bypass, poisoned completion results, or schedule pausing, with no identified disclosure of Workflow payloads or history.