Propagate request traces through derived Maven sessions
Build-platform observability feature: Maven integrations can now carry typed request-trace context through derived sessions and Resolver operations without losing nesting or thread isolation.
Added propagation, nesting, repository derivation, concurrency, null-cleanup, explicit-restoration, install, and deploy regressions; all 21 focused tests passed.
The full Maven verification build passed. Core IT reported 1,059 passing tests under JDK 26; the two JDK-sensitive failures were rerun under JDK 21 and all four affected test methods passed.