fix(core): clean up mcp stdio descendants - #34525

Summary

• terminate descendant processes for stdio MCP transports before closing the SDK client or transport
• reuse the cleanup path for failed startup transport teardown
• keep the change scoped to local MCP process cleanup

Verification

• inspected MCP SDK v1.29.0 StdioClientTransport.close: it only closes/kills the root server process
• confirmed with a local POSIX repro that a child process can survive after its parent is killed
• git diff --check
• bun typecheck could not run because tsgo is not installed in this checkout