feat: add web-session pool observability (MCP tool + health-matrix) - #3395

Review failed: Failed to connect to ingest: wss://cloud-agent-next.kilosessions.ai/sessions/oauth%2Fgoogle%3A111280339471504352511/agent_d41419fb-394d-4f6d-b103-6e98e009d567/ingest?wrapperRunId=wr_c0c4b9ad912d4b0ebb21fc8c25556a51&kiloSessionId=&sessionId=agent_d41419fb-394d-4f6d-b103-6e98e009d567&wrapperGeneration=3&wrapperConnectionId=90e015c2-3f53-437c-b9a3-376ebfd35d9d (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.)