[M2] Implement JSON-RPC message framing

[zacky1972]

Summary

Implement the JSON-RPC framing used by Language Server Protocol over stdio.

Tasks

• Encode messages with Content-Length headers.
• Decode incoming byte streams into complete JSON-RPC messages.
• Handle partial frames and multiple frames in one buffer.
• Add unit tests for framing edge cases.

Acceptance criteria

• Encoder and decoder round-trip valid JSON-RPC payloads.
• Decoder preserves incomplete buffers until a full frame is available.