Fix JsonEncode treating tables with index 0 as arrays - #454

…(shutdown hang), the

latter guarded for combined-host mode

BeamMP#454 -- Util.JsonEncode treated ANY numeric key as an array index, so a plugin
table like {[0]="a",[1]=true} encoded as the bare array ["a",true] and the 0 key
vanished silently: the array branch push_back()es values and drops keys. Zero,
negative and fractional keys all hit it. A table is now an array only when every
key is an integer >= 1 (IsLuaArray), applied at both call sites.

BeamMP#501 -- GracefullyShutdown could run every subsystem handler and then hang
forever on a lingering thread or a static destructor, leaving a server that had
"shut down" but never exited. It now _Exit(0)s once the handlers have run
(_Exit deliberately skips atexit/static destructors -- that is what makes it
immune to those hangs).

FORK GUARD on BeamMP#501: only for the STANDALONE server. In combined-host mode the
server is a library inside the launcher process, so exiting there would kill the
launcher and the host's own game session with it. Same reasoning as the existing
hard-shutdown guard a few lines above, which already checks IsEmbedded().

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>