REGTESTS: filters: Exercise stream isolation across backend modes

REGTESTS: filters: Exercise stream isolation across backend modes

Reuse a client connection while switching between instance-mode, legacy
and fully disabled backends, then open another client connection. Check
that every transaction gets new instances, construction follows global
order, response order stays independent, and no backend links or state
leak into a later request.

Include disabled instances, declined attachment, request and response
payload filtering with small forwarding quotas, and a chunked request.
Validate request byte counts and bodies as well as callback order. Reuse
the instance backend's connection across transactions and clients.