Tool registry transforms and request snapshots

Keep one stable MCP Tool.Service.transform that reads the latest discovered-tool source. Refresh the source and reload state instead of re-registering MCP tools, which would move them after plugins and steal override precedence. Each model request must snapshot both advertised definitions and executors so execution honors exactly the contract shown to that request. Reloads and disposal affect only later snapshots; disposal rebuilds transforms and reveals earlier overridden registrations. Captured executors may still observe producer-owned mutable state.