Check agent MCP-server health

Returns a per-MCP-server health summary for the agent. Each MCP server referenced by the agent (its tools.json plus every subagent's tools.json) is probed in parallel by issuing a single MCP initialize JSON-RPC call. Credentials are resolved per (tenant, server) using the configured authentication method. Per-server failures surface as ok=false with an error_code (auth_failed | unreachable | invalid_response) and a human-readable error string. mcp_check.ok is true only when every server returns a healthy response; an empty server list is treated as healthy.