Constant MAX_STDIO_JSONL_LINE_BYTES 

const MAX_STDIO_JSONL_LINE_BYTES: usize = _; // 10_485_760usize

Max bytes accepted for one inbound stdio JSONL frame.

This caps only client -> server request lines. Server -> client responses are still written normally through stdout. 10 MiB matches the REST strict JSON body order of magnitude and leaves plenty of room for large tool args while preventing an unbounded Vec growth on malformed/no-newline input.