Fix: Buffer stdout data to handle large/fragmented JSON responses

[linacs]

This PR fixes an issue where the MCP client would fail to parse large or fragmented JSON responses from the server. Previously, the code assumed that each chunk received from stdout contained a complete JSON message, which is not guaranteed by Node.js streams. As a result, long responses could be split and cause parsing errors or incomplete logs.

Changes:

Added a buffer to accumulate data from stdout until a full message (ending with \n) is received.
Only complete messages are parsed and processed, preventing errors with large or split responses.
Why:
This change ensures robust handling of server responses, especially when dealing with large payloads, and prevents log truncation or JSON parsing errors.