Fall through when the tray has gone

[SimonCropp]

PendingFiles routes on DiffEngineTray.IsRunning, which a type initialiser reads
once. CLAUDE.md documents the late-starting tray - a tray that appears after the
test host, which the queue owner branch already handles. This is the mirror
case, and nothing handled it.

A tray that exits while a long lived host keeps running leaves that cached
answer saying a tray is there. The piper send then went to a port nobody was
listening on, and returned void: the refusal was swallowed into a trace line and
the caller carried on believing it had handed the file over. Every later move
and delete for the life of that process was pending in nothing at all - no
fallback to the queue owner, and no LaunchDelete either.

Have the piper send report whether it connected, and treat a refusal the same as
no tray. That is the branch that already exists and already knows what to do.