RPC Message Refactor + JSON RPC Error Codes

[patvice]

• Replace the bespoke request/response classes with a consolidated RubyLLM::MCP::Native::Messages module that emits JSON-RPC 2.0 envelopes and validates every request/notification against the official 2025‑06‑18 MCP schema (vendored fixture + json_schemer coverage).
• Introduce Native::JsonRpc helpers and an envelope validator so transports (stdio, streamable_http, sse), ResponseHandler, and Result all speak consistent JSON-RPC, surface canonical error codes, and correctly detect notifications vs. responses.
• Expand specs for the new message builders, JSON-RPC validation, transports, and results while deleting the old bespoke request/response objects, ensuring the client now conforms to the MCP spec end-to-end.