fix(orchestrator-core): pin environment host across prepare/exec/teardown

Environment plugins are stateful across the three-call contract: prepare
registers an in-memory run (e.g. a live WS relay to a remote node) that
exec/exec_stream/teardown reuse by handle. The previous per-call
resident-host lease dropped between calls, letting the shared registry
LRU-evict or ping-reap the process between prepare and exec — losing that
state and surfacing as "no live relay connection for handle".

EnvironmentClient now holds ONE ResidentHostLease for its lifetime
(acquired lazily on the first RPC via pinned_host), so every RPC lands on
the same pinned process. The client is now non-Clone (callers Arc it) so
the single lease is not duplicated.

Death handling splits by call kind: exec/exec_stream stay at-most-once
(no retry — side effects + lost state), while the idempotent control ops
prepare/teardown retry once. Either way a dead lease is reaped
generation-scoped (drop from the client + invalidate the exact generation
in the registry) so the next run spawns a live process instead of
re-leasing the corpse, and a concurrent call's fresh lease is never
evicted by a late failure of an older generation.

Removes the now-unused with_resident_host / with_resident_host_once /
ping_is_dead helpers and their ping tests.

Author: Shooksie