fix: buffer stdout in SimpleStdioMcpClient for split/batched JSON-RPC messages

[anuragg-saxenaa]

Fixes #276

What

Adds proper newline-delimited JSON buffering to stdout parsing, replacing the naive single-message-per-data-event assumption.

Why

Node.js stream events do not guarantee message boundaries. The previous implementation called directly on each raw event, which:

1. Crashes when a JSON-RPC message is split across multiple events (partial JSON)
2. Drops messages when multiple newline-delimited JSON messages arrive in a single event (only the first would be parsed)

This is a well-known pitfall with Node.js child process stdout handling and becomes more likely with larger payloads or rapid-fire MCP tool responses.

Changes

• Added a stdoutBuffer field to accumulate incoming data across data events
• Split buffered data on newlines, keeping the last (potentially incomplete) segment in the buffer
• Parse each complete line individually with error handling (try/catch + Log.warn)
• Reset buffer on reconnect to prevent stale data from a previous session