Use AHP ping for active liveness on remote agent host connections (#316358)

* Use AHP ping for active liveness on remote agent host connections

Replaces the previous per-request timeout with a periodic ping that
keeps the connection alive against idle-timeout intermediaries and
detects a dead transport by force-closing after 20s of silence. The
service then schedules a reconnect via its existing exponential
backoff. Mirrors the keep-alive pattern in PersistentProtocol
(KeepAliveSendTime / TimeoutTime) and reuses LoadEstimator to suppress
the close when local CPU is pegged.

Notes:
- The protocol-side ping command lands separately in
  microsoft/agent-host-protocol#119.
- Tolerates servers without ping support: a MethodNotFound response
  is itself inbound traffic, so liveness is satisfied.

(Written by Copilot)

* fix

* Close transport in ping-before-initialize test to release server-side disposables (Written by Copilot)