Stop the frontend tree when headless reflex run exits
Merged
FL
13d ago+498+498+012345678901234567890123456789-19-19-01234567890123456789
Closes #6980.
When the run unwinds, _run_dev terminates the frontend process. The frontend stays in the CLI process group, so a SIGKILL of the group also stops it. A frontend that launches after shutdown starts is terminated at launch. On POSIX, frontend-only mode exits cleanly on SIGTERM. This replaces #7209 with a smaller change.
Tests: no-TTY subprocess cases for SIGTERM, SIGINT and SIGKILL in frontend-only and full-stack mode, plus a failing frontend task, a normal backend return and a late frontend launch. Windows descendant cleanup is not changed.