JLSEC-2026-621 Thread-safety and out-of-bounds reads in the HTTP.jl content-type sniffer

Description The content-type sniffer had two memory/concurrency bugs. Number sniffing in isjson stored the strtod end-pointer out-parameter in a shared module-global VectorPtrUInt8; concurrent sniff calls on a multithreaded server raced on that single cell between the ccall and the subsequent rea...