Introduce JSON-RPC 2.0 handling and a minimal stdio framing server. Adds ApiEndpoints/JsonRpc2 and JsonRpc2Dispatcher to parse and dispatch JSON-RPC requests, and StdioServer to handle Content-Length framed stdio messages. Adds McpToolsList.json (tools list resource) and wires JsonRpc2 into ResourceServer.
Program.cs: add --stdio-jsonrpc2 mode to run the stdio server, and integrate dispatcher usage (with timeout and TODO placeholders for tools/call). ResourceServer: register JsonRpc2 endpoint, change fallback handling to 404 via ServeResource(), and make several resource helper methods static/public for reuse. Update csproj to include new source files and embed the McpToolsList.json resource.
Contains basic implementation and scaffolding; tool-call handling is left as TODO.
