Same attached-cell guard as the local kernel host (#101861): a remote
kernel mid-cell is never reaped or cap-evicted, so a fan-out never has
its runner killed under a live poll loop.
Local session kernels sweep idle-expired entries and enforce a
process-wide cap (DEFAULT_MAX_SESSION_KERNELS) on every call
(tools/code_kernel.py's _reap_unlocked / _evict_over_cap_unlocked). The
new remote kernel host (#96991) never got the same treatment:
_REMOTE_KERNELS only shrinks lazily when a specific key is revisited and
found dead, so an owner that opens kernels for several distinct
(env_type, task_env_id) combinations (or delegated children) and never
revisits some of them accumulates host-side bookkeeping entries for the
life of the gateway process.
Note this is narrower than the local case: the remote runner already
self-reaps on its own idle timeout, and SSH/Docker connections are
independently bounded by their own transport-level lifecycles (SSH
ControlPersist, Docker's session-scoped idle-timeout in terminal_tool.py)
— so nothing here leaks a live remote connection. What's missing is
purely the host-side dict/cap bookkeeping symmetry with local kernels.
Adds _reap_unlocked/_evict_over_cap_unlocked mirroring the local
implementation, reusing the same max_session_kernels config as an
independent cap on _REMOTE_KERNELS.
