Stdio and ByteStreams buffer unbounded incoming lines #342

Stdio and ByteStreams frame newline-delimited JSON with futures::AsyncBufReadExt::lines(). A peer can send an unterminated or oversized line and grow the process without a finite ceiling before JSON-RPC parsing.

This is the server-side counterpart to #340: ACP/MCP tools commonly expose Stdio::new() directly to an agent process.

Expected: a generous finite default, explicit per-transport override, bounded buffering before UTF-8/JSON parsing, and transport failure on overflow.