Release transport resources. When this transport owns a spawned subprocess
(McpClient.spawn), run the MCP stdio Shutdown sequence (basic/lifecycle
§Shutdown -> stdio): close the child's stdin, escalate to SIGTERM, then
SIGKILL if it does not exit within the grace periods. A no-op when there is
no owned subprocess (a custom readLine/writeLine channel).
Release transport resources. When this transport owns a spawned subprocess (McpClient.spawn), run the MCP stdio Shutdown sequence (basic/lifecycle §Shutdown -> stdio): close the child's stdin, escalate to SIGTERM, then SIGKILL if it does not exit within the grace periods. A no-op when there is no owned subprocess (a custom readLine/writeLine channel).