feat: add REST API for session pool health (dashboard interface) - #3404

Review failed: Failed to connect to ingest: wss://cloud-agent-next.kilosessions.ai/sessions/oauth%2Fgoogle%3A111280339471504352511/agent_71ae3858-e8cb-4b7e-bbb7-da39791d56ab/ingest?wrapperRunId=wr_20762a390ba246a9ab82330c858c2e47&kiloSessionId=&sessionId=agent_71ae3858-e8cb-4b7e-bbb7-da39791d56ab&wrapperGeneration=3&wrapperConnectionId=57e80bc7-326e-4388-829c-24e0661e995b (WebSocket error before open; Bun does not expose the HTTP status. If the Worker logs no /ingest request, check WORKER_URL and sandbox-to-host networking; if it logs a 4xx, inspect the DO rejection reason.)