TUN-10820: Remove test-only stdin reconnect control

Remove the hidden stdin-control flag and ReconnectSignal plumbing used to
force tunnel reconnections through stdin. Delete the associated component
test and supervisor control implementation.

Serve QUIC and HTTP/2 connections directly with the caller context instead
of wrapping them in an additional errgroup for reconnect signals. Automatic
reconnection after genuine transport failures remains unchanged.

This removal should help simplify the supervisor loop a bit and make way
for better improvements in this area.

Closes TUN-10820