Add disconnect hooks for MCP websocket servers

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Summary

• add an OnDisconnect hook to websocket server configuration
• invoke disconnect cleanup when MCP websocket sessions close
• support MCP-only websocket servers used by AgencyProxy

Release-family impact

This is the endpoint-libs gate required by AgencyProxy. Do not publish or merge it in isolation: after approval, release endpoint-libs first, then update honey_id-types and endpoint-gen, then bump both endpoint-libs and honey_id-types together across the six backends according to docs/release-order.md.

Verification

• cargo test: 58 passed, 0 failed
• ./scripts/check-chain.sh --quick: only the two documented known-red family conditions remain (EndpointValidator has two endpoint-libs majors; honey_id-types has an unpublished local bump)